Intel CPUs

Disc images (commonly known as ISOs, but ISO is actually a specific format) are faithful software recreations of game discs (when made correctly). However, with disc sizes ranging from 700 MB (CD), 1.4 GB (GC Mini-DVD), 4.7 GB (single-layered DVD), and 25 GB (Blu-Ray), they can get pretty taxing for storage, especially when newer generations of consoles games are getting bigger in file sizes.

It wouldn't be so bad if not for the fact that the game data itself is often times only a fraction of the actual disc size - for instance, the Super Mario 25th Anniversary Wii disc itself is a 4.7GB, when really the actual game data is only a single SNES ROM (12 MB of useful data, to be precise) and nothing else. Naturally, one would want to trim this extra "fat" as much as possible, which is what this page aims to help to achieve. Most of the information here is based partially on this guide.

How does one lighten ISO / ROM dumps?

There are many ways, some methods alter the data forever while others can be converted back and forth with generally no loss. Some conversions are only playable on specific emulators and may not work on real hardware depending on the console and the method used. It's important to take all this into consideration before attempting as most of these are console-specific.

Archive-quality dumps are ones that when converted back to its original state, will have the same checksum as the official uncompressed release. Compressions that can't be reversed, or those that can be but will have missing or altered content whether it interferes with functionality (rebuilt table of content) or not, are not archive-quality. For example, the WBFS format, used for shrinking Wii discs, is not archive-quality since it may be missing padding content and upgrade partitions (which have their uses in 3DS/Wii modding) compared to an intact, uncompressed dump.

Applicable to All Platforms

Audio-CD

PCs, Sega-CD, PC-Engine, PlayStation, Sega Saturn... what do these systems all have in common? They all use a regular CD format! Game developers often stored music and other sounds using the Audio-CD format, but it was terribly inefficient when it comes to disc storage as it also had to store the actual game along with the sound files (commonly known as a "mixed CD"), and these sound files are never compressed because the Audio-CD standard simply doesn't allow for sound compression. To put it in perspective, a 700 MB CD containing nothing but Audio-CD data can hold at most around 80 minutes worth of sound data, meaning games that used a lot of sounds were limited in size.

Because these mixed CDs are difficult to properly archive just by using standard .ISO files, data-dumping software will usually do one of two things as a workaround:

Full Dump: BIN/ISO + CUE

• BIN/ISO is the full disc data, including Audio-CD sound data and game data
• CUE is the datasheet file

Light Dump: ISO + MP3/WAV + CUE

• ISO is the disc data with only the game data
• MP3/WAV is the sound data from the Audio-CD, but these formats take much less disk space
• CUE is the datasheet file

Developers have long since stopped using the Audio-CD format, and instead prefer custom audio formats that come included in the "game data" part of the disc. By the launch of the fifth generation CD-based consoles, i.e. the PS1 & the Saturn, it was more common than not to just use the Audio-CD part for messages like "Don't put this in a CD player!" and little else. That being said, there were still quite a few fifth-gen games that used Audio-CD data for their soundtracks, such as Vib-Ribbon and the Wipeout trilogy on the PS1, and Battle Garegga and Daytona USA on the Saturn.

• Archive-quality dump? No (unless audio is converted to and from uncompressed formats, which is unlikely)
• Gain: Several hundreds of MBs to just a few dozens, depending on how much this specific game relies on the Audio-CD sound format
• Tools Used: Load the BIN+CUE using a virtual drive, then use a CD dumping tool
• Can be reverted? Yes, just burn the ISO+MP3/WAV+CUE again using a CD burner tool (ImgBurner) either to a physical disk or as an ISO+BIN file. Lossy audio formats will result in data loss.
• Playable on Hardware? No, but can be reverted to be
• Playable on Emulators? Yes (use virtual drive if needed). Some aren't compatible with MP3, if that's the case, convert them to WAV with MP32WAV. You may need Sega Cue Maker.

Examples:

• Princess Crown (SAT): 574 MB > 72 MB (Game Data) + 50 MB (Audio-CD data in MP3 format)
• Captain Tsubasa (SCD): 512 MB > 146 MB (Game Data) + 3 MB (audio as MP3) > (as 7zip) 34 MB (Game Data) + 3 MB (audio)

M3U (playlist) files may be used too for this distribution scheme.

Sometimes dumps that come this way may not work on some emulators. This is often due to either an incorrect CUE files using the wrong filenames or using MP3 instead of WAV.

Padding

Devs often have their games much, much bigger than they need to be by putting in accessible garbage data in the disc. Garbage data isn't useful game data and is just used to bloat the disc size. It's either a sequence of 00/FF (you know what's inside a file if you open it with a hex editor), random data, unused/cut content left during development, and in rare cases, data that is completely unrelated to the game itself. An example of this is Shrek SuperSlam on the PS2 which has a working copy of Tony Hawk's Underground 2 for the PSP hidden in the root of the disc.

The purpose of doing this can be to fill in spots on the disc so that specific parts of the game's data are in certain areas of the disc. This is done to increase the drive's reading speed so that it's quick enough in certain spots for the game to work properly. It's in the your best interest not to mess with this data arrangement (referred to as LBA and TOC in the case of GC/Wii/PS2/PSP) as it might break the game in some cases.

Another reason for having garbage data can be to screw with pirates, who download/upload these games online, by making the ISO bigger and harder to store. Some go a step further and scramble the garbage data, instead of just being a sequence of 00/FF, to make the ISO much harder to compress using regular archive formats like zip, 7zip, rar, etc. You might be overjoyed to learn this has become the industry standard nowadays.

Many compression schemes remove or simplify padding patterns to allow for easier compression.

CHD Compression

• Archive-quality dump? Yes (with CHD version 5)
• Gain: Immediate
• Tools Used: chdman (included with MAME)
• Can be reverted? Yes, using extractcd (included with MAME)
• Playable on Hardware? No.
• Playable on Emulators? MAME, DuckStation, PCSX2 (since March of 2021) and DEmul. Some libretro cores for other emulators are starting to add support.
• Can process multi track bin files? Yes.

MAME uses the CHD format for disc images in general and includes tools to convert back and forth. Before MAME v145, CHD was in version 4 and it bumped to version 5 from MAME v146 and further. The CHD v5 uses 7zip's LZMA compression on the game data and lossless FLAC compression for the audio data to optimize compression even further than using BIN+CUE+MP3/WAV data separation alone. CHDv5 is truely lossless.

Instructions Place chdman.exe and extractcd in the same directory as the dumps you want to compress (dumps must be in BIN+CUE format or GDI). Open Command Prompt and navigate to the directory where you placed chdman.exe and input one of the following:

• BIN/CUE-GDI to CHD: for /R %i in (*.cue, *.gdi) do chdman createcd -i "%i" -o "%~ni.chd" (Windows)
• CHD to BIN/CUE: for /R %i in (*.chd) do chdman extractcd -i "%i" -o "%~ni.cue" (Windows)

Alternatively, if you only need to do one file you can use this: chdman createcd -i "<FILENAME>.cue" -o "<FILENAME>.chd"

If you have one of the European PSX games that features LibCrypt copy protection, then you will have a .sbi file in addition to the .bin/cue file. You will still need to have the .sbi file in the same directory as the game file (in this case, the newly created CHD file) in order to run.

PlayStation 1

ECM

• Archive-quality dump? No (removes error correction data permanently)
• Gain: Not Immediate (ISO size doesn't change). ECM only zeroes out redundant error correction data, but in some rare cases this data may be used for anti-piracy, hence corrupting the dump! However it does achieves drastic size reductions when compressed to an archive format (7zip/gzip/zip) .
• Tools Used: PakkISO or ECM Tools. Drag-and-drop the ISO onto the specified tool, then compress it with 7zip.

• Can be reverted? Yes, using the same tools (unECM), however there is data loss (error correction data) which can damage a select few games. Check the hash with Redump to make sure nothing was altered.
• Playable on Hardware? No.
• Playable on Emulators? Only on recent SVN builds of PCSX-R and ePSXe (they also support 7zip/gzip/zip archives so use them with ECM).

PBP (PSP Format for PS1 Images)

• Archive-quality dump? No
• Gain: Immediate (ISO size decreases a lot).
• Tools Used: PSX2PSP, Popstation MD GUI, CDDA-ENABLER (optimized for PS3) ...

• Can be reverted? Yes. For homemade EBOOTs it can be done by using the same tool to generate the BIN+CUE files. For commercial releases from PSN you will have to use PSXtract. There's data loss, but it's negligible in terms of functionality.
• Playable on Hardware? On PSP, not on PS1.
• Playable on Emulators? PCSX-R, DuckStation and ePSXe. PlayStation 3's ps1_netemu can also use it after packaging it into a PS1 Classic and installing said package.

PBP is the official format used by Sony for the PS1 Classics on PSP and PS3. Audio tracks may be compressed in ATRAC3 or as raw PCM (unsupported on PS3's ps1_netemu, supported on PSP only by the earliest versions of POPS or the newest ones with the cdda_enabler plugin)

CHD

• Archive-quality dump? Yes
• Gain: Immediate (better than PBP).
• Tools Used: chdman
• Can be reverted? Yes, using the same tool (chdman).
• Playable on Hardware? No.
• Playable on Emulators? libretro Beetle PSX, Avocado, DuckStation, PCSX2 (since March of 2021, and also because PCSX2 now has PSOne emulation functionality as well, so two-in-one) and PCSX ReARMed.

PlayStation 2

Manual Scrubbing + GZIP Compression

Often times you can just open the ISO in UltraISO and find the dummy files. Sometimes they are obvious looking files, like DUMMY.BIN, DATA0.BIN (or .DAT), DUMMY.DAT, etc., or are folders with names like "PADDING" and such. You can look into the files (with a hex editor) to see if they're obviously padding data (usually the offsets will be full of 00/FF, though sometimes it's not as obvious). However, you must never mess with LBA and TOC when removing padding.

• Archive-quality dump? No (missing data)
• Gain: Not Immediate (ISO dump size the same), however when used with compression the gain is really noticeable.
• Tools Used: UltraISO, USBUtil, ExPERT/Xpert (guides here)
• Can be reverted? Not really. Garbage data here is zeroed out, it's not important and you still have the same disk data structure.
• Playable on Hardware? Yes (after GZIP-decompression) (some games may break)
• Playable on Emulators? Yes - PCSX2 (some games may break)

You may forego the manual scrubbing part entirely. If you feel adventurous you might want to do it to enhance compression though. Let's get to the meaty part though...

The PCSX2 emulator supports opening compressed archives containing ISOs. The best format it supports would be GZIP.

Use 7zip ("Add to archive..." then choose to gzip) or Pigz (multi-threaded, much faster compression) to generate gzip archives containing the ISO file in question. PCSX2 will build an index of each gzip compressed game it loads (as a file in the same directory as the gzip archive), so after the first time where you'll have to wait for the decompression, in all subsequent times there is no speed difference between playing an uncompressed and compressed game. Of course, you can extract the ISO back from the GZIP archive.

A simple method to mass convert a lot of PS2 ISO games to GZIP is this script when run in PowerShell. However, it'll take many hours or days depending on the size of the collection.

dir *.iso | ForEach-Object { & "C:\Program Files\7-Zip\7z.exe" a -tgzip -mx9 -sdel ($_.Name+".gz") $_.Name }

CSO (aka CISO)

• Archive-quality dump? No (missing data)
• Gain: Immediate (ISO dump size the same). Coupled with manual scrubbing, it can be bigger. Not as much of a gain as GZIP though.
• Tools Used: maxcso
• Can be reverted? Yes, using the same tool. No data loss.
• Playable on Hardware? NO
• Playable on Emulators? Yes - PCSX2.

CHD

• Archive-quality dump? Yes
• Gain: Immediate (better than GZIP).
• Tools Used: chdman
• Can be reverted? Yes, using the same tool (chdman).
• Playable on Hardware? No.
• Playable on Emulators? PCSX2 (as of Q1 2021).

PlayStation 3

Extracted files (aka JB format/GAMES)

• Archive-quality dump? Not directly (BD filesystem metadata lost).
• Gain: Immediate (any padding between files is removed).
• Tools Used: Most on-console file managers and *man homebrews, 7-Zip and other standard ISO extractors on PC.
• Can be reverted? Yes, using PS3 ISO Rebuilder and an IRD file representing the original file locations. Generic inaccurate JB to ISO conversions possible with makeps3iso (preferred) or genps3iso.
• Playable on Hardware? Yes - on CFW/HEN
• Playable on Emulators? Yes - RPCS3

For a long time (before the availability of Cobra and Mamba) the only playable format for PS3 backups (relying on little more than "peek and poke" CFW-provided syscalls to mount the game's folder over the currently inserted disc) but also the least accurate one, with significant compatibility issues (varying on whether the backup is stored on an internal or external drive, whether an original game disc is currently inserted, various optional hacks such as "BDMirror" moving the files to the root of the external drive on demand, ...)

PlayStation Portable

CSO (aka CISO)

• Archive-quality dump? Maybe: while the actual compression is inherently lossless, depending on the conversion software and/or its configuration, so-called ripping (deletion or zero-resizing of the system update, videos, or other files) may be performed on the fly.
• Gain: Immediate (ISO dump size lowers). Substantial, depending on game. Higher compression levels generally produce decreasing space gains while potentially increasing load times. Some tools allow for leaving audio/video content uncompressed.
• Tools Used: CISO, CISO GUI, maxcso, UMDGen, PSP ISO Compressor, CISO Multi Compressor, CISO XP, ...
• Can be reverted? Yes, using the same tools and saving in ISO (uncompressed) format. No data loss.
• Playable on Hardware? Yes - most CFWs (Possibly longer load times though).
• Playable on Emulators? Yes - PPSSPP.

CISO GUI and CISO XP: Drag-and-drop ISO file on program, Select compression level (1 to 9 (Max)), "Compress", and choose directory for saving the new CSO file.

UMDGen is a PSP ISO editor. You can save to either the CSO format with this.

DAX, JSO, ZSO

Three different compressed-ISO formats. Significantly lower popularity/support than CSO.

PBP

• Archive-quality dump? ???
• Gain: Immediate (ISO dump size lowers).
• Tools Used: ISO~PBP Converter, or Sign~Fake NP Expert
• Can be reverted? Yes
• Playable on Hardware? Yes - CFWs (and, if signed, OFWs too).
• Playable on Emulators? Yes - PPSSPP.

Official format for digitally distributed PSP/minis (partially related to the one used for PS1 games), containing a compressed disc image and optionally a custom boot logo.

PlayStation Vita

Trimming

• Archive-quality dump? No, but trivially undoable.
• Gain: Immediate (ROM size lowers).
• Tools Used: None - manual process
• Can be reverted? Yes (see above).
• Playable on Hardware? Yes - with psvgamesd.
• Playable on Emulators? ?

The psvgamesd gamecard backup creation and mounting tools supports mounting a trimmed backup (although no tools exist for dumping directly in trimmed format, or converting between full and trimmed.

Neither variant of the .psv format (unrelated to the Cobra Blackfin's .psv format) has caught on, despite being the most accurate options for physical game backups.

Extracted files (NoNpDRM format)

• Archive-quality dump? No for physical titles, close enough (and generally accepted as such) but no cigar for digital ones.
• Gain: Immediate (compared to ROM).
• Tools Used: NoNpDRM plugin + a file manager like VitaShell.
• Can be reverted? No.
• Playable on Hardware? Yes - with the NoNpDRM plugin.
• Playable on Emulators? ?

NoNpDRM is a Vita plugin that generates decrypted licenses (valid for any console) when running an original Vita title, as well as allowing the system to accept those licenses, allowing for playing the encrypted files directly copied from a game card or memory card (by extension the name is therefore also used for such files, or the combination of the game's files and a decrypted license).

NoPsmDRM is the equivalent for PlayStation Mobile titles.

An hypothetical NoPspEmuDRM, allowing for PSP/PS1 eboots to be natively runnable and manageable from the LiveArea/Content Manager, is currently not known to exist nor believed to be in development.

Dreamcast

CHD Archive Format

• Archive-quality dump? CHD v4: No (missing data due to lossy compression, no gameplay issues). CHD v5: Possibly Archive-quality.
• Gain: Immediate (ROM dump size lowers).
• Tools Used: GDI to CHD converter.
• Can be reverted? Yes with CHD v5.
• Playable on Hardware? No
• Playable on Emulators? Yes - DEmul, Reicast, Flycast, and Redream

GameCube / Wii

People used to resort to WiiScrubber (Wii) and GCM Utility (GC) to scrub/trim games to end up with dumps that while they had no immediate size change, their randomized garbage data (like "dummy", "padding" or "znull") was still there but zeroed out making archived dumps using zip/7zip/rar formats have stunning gains (from 1.4GB uncompressed to 26MB zipped for Animal Crossing for example!). Of course, the file still needed to be uncompressed to its full size everytime you wanted to play it.

Trimming and scrubbing (in Wiiscrubber terms) aren't the same! While they both are terms for "zeroing garbage data" to make it more compression-friendly, trimming does not just that like scrubbing but takes the extra step of relocating the garbage data to the end of the file, hence altering its TOC and requiring the disc to be fakesigned, for a not-so-big compression gain. Hence why scrubbing is by far the most authentic and safe way to solve the garbage data problem.

However compressed formats incorporating the "padding zeroing" part were made since then, and Dolphin supports them! These are the WBFS and GCZ formats.

GCZ (Dolphin native archive format) - GC/Wii

Dolphin can't play games directly from compressed archives (7zip/zip/gzip/rar...). Instead, it utilizes its own compression method. This format has been deprecated in favor of RVZ.

• Archive-quality dump? Not directly (can be restored with NKit).
• Gain: Immediate (ISO dump size lowers). Considerable, depending on the game.
• Tools Used: Dolphin (right-click the game(s) in the games list and select "Compress ISO..." (or "Compressed selected ISOs..." if more than one is selected)), NKit, or wit (Wiimms ISO Tools)
• Can be reverted? Yes. You can right-click on the GCZ-compressed (in blue) ISOs in Dolphin and choose "Uncompress selected ISOs". Garbage data for Wii titles will be zeroed out and thus different from the official dump but will have no effect on gameplay in most cases - if desired can be unscrubbed NKitRestore.
• Playable on Hardware? No
• Playable on Emulators? Only Dolphin.

Some examples:

• Super Mario Anniversary (Wii): 4.7 GB (original) > 12 MB (GCZ) (!!)
• Super Smash Bros Brawl (Wii): 7.8 GB (original) > 7.1 GB (GCZ) (main cause is FMV's low compression)
• Xenoblade PAL (Wii): 7.8 GB (original) > 6.3 GB (GCZ)
• Super Mario Galaxy 2 (Wii): 4.7 GB (original) > 1.8 GB (GCZ)
• Tales of Graces (Wii): 4.7 GB (original) > 4.2 GB (GCZ) (game data already fills the disk)
• Animal Crossing (GC): 1.4 GB (original) > 26 MB (GCZ)
• Zelda Four Swords Plus Japan (GC): 1.4 GB (original) > 480 MB (GCZ)
• Zelda Twilight Princess (GC): 1.4 GB (original) > 1.4 GB (GCZ) (game data already fills the disk)
• Megaman Collection (GC): 1.4 GB (original) > 1.1 GB (GCZ) (sound data is stored as uncompressed stream to fill disk)

RVZ (Modern Dolphin format) - GC/Wii

The Dolphin team developed a new compression format based on WIA called RVZ. Unlike all the previous formats, RVZ is lossless and can preserve the padding data on Wii discs as well as the necessary files needed by the Wii's IOS. Usage is very similar to GCZ in Dolphin itself, but it only works on newer Dolphin versions.

• Archive-quality dump? Yes.
• Gain: Immediate (ISO dump size lowers). Considerable, depending on the game.
• Tools Used: Dolphin 5.0-12188+ (right-click the game(s) in the games list and select "Compress ISO..." (or "Compressed selected ISOs..." if more than one is selected))
• Can be reverted? Yes.
• Playable on Hardware? No.
• Playable on Emulators? Only Dolphin.

Scrubbing and Trimming - GC/Wii

• Archive-quality dump? Not directly, see note about reversing.
• Gain: Not Immediate for just scrubbing (ISO dump size the same); Immediate for trimming (ISO dump size lowers).
• Tools Used: GC-Tool ("wipe garbage data..." - Scrubbing for GameCube), GameCube ISO Trimmer (scrubbing and trimming for GameCube), WiiScrubber (scrubbing/trimming for Wii, although compressing with Dolphin will scrub it anyways).
• Can be reverted? Yes, with WiiScrubber's undo file or NKit.
• Playable on Hardware? Yes for scrubbing; some games won't work trimmed.
• Playable on Emulators? Yes for scrubbing; some games won't work trimmed.

Scrubbing games zeros out garbage/dummy data in the ISO file. The resulting file will be the same size, but it will be able to compress better than unmodified ISOs. The difference can be huge depending on the game. Animal Crossing, for instance, will compress to just a 26 MB .gcz file after being scrubbed!

Trimming (also known as "trucha scrubbing", and substantially different from the definition of "trimming" used in ROM-based systems) games will also defragment the files moving them towards the start of the disc image, resulting in a smaller ISO file. The trimmed file can be used immediately at a smaller size without compression, but the game's internal structure will be wholly modified. Games relying on direct sector access (as opposed to consulting the filesystem) will break if you trim them.

WBFS - Wii only

• Archive-quality dump? Not directly, see note about reversing.
• Gain: Immediate (ISO dump size lowers). Considerable, depending on the game.
• Tools Used: Wii Backup Manager (after adding your game, go to "Transfer" and select "WBFS") or wit (Wiimms ISO Tools).
• Can be reverted? Maybe. WBFS files can always be converted back to ISO, but they will remain scrubbed (can be unscrubbed with NKitRestore) and, depending on settings at the time of conversion to WBFS, may be missing update/extras partitions (can be readded with wit or NKitRestore and a copy of the missing data).
• Playable on Hardware? Yes - Preferred format by most SD/USB loaders
• Playable on Emulators? Yes - Dolphin

A scrubbed and sparse (gap-dropping) format directly derived from the one used in the obsolete WBFS file system.

Ideal space-saving option for Wii games to be played via an USB loader.

CSO (aka CISO) - GC/Wii

• Archive-quality dump? Not directly, see note about reversing.
• Gain: Immediate (ISO dump size lowers). Less than WBFS.
• Tools Used: Wii Backup Manager (same as above, but select "CISO" - possibly Wii only), wit
• Can be reverted? Maybe (same caveats as WBFS).
• Playable on Hardware? Yes (on Wii/Vwii only) - Nintendont for Gamecube games, uLoader for Wii games
• Playable on Emulators? Yes - Dolphin

A scrubbed and sparse (gap-dropping) format. Unrelated to the PSP's CSO format.

Great space-saving option for Gamecube games to be played on Nintendont.

FST (extracted File System) - GC/Wii

• Archive-quality dump? No
• Gain: Very low with GC ISOs, much better with Wii ones. Can also be combined with classic archives compression (ZIP/RAR/7z etc.) to match WIA and NKit compression ratios.
• Tools Used: wit
• Can be reverted? Partially via the align-files.txt created by wit when extracting.
• Playable on Hardware? (GC games on Wii/Vwii): Yes - Nintendont. (Other combinations): No
• Playable on Emulators? Yes - Dolphin (both GC and Wii)

This has the advantage of easy experimenting with #Destructive Modification. Includes all disadvantages of trimming.

WIA (Wii ISO Archive) - Wii only?

• Archive-quality dump? Not directly unless --raw option is given, effectively preventing any space savings.
• Gain: Immediate (ISO dump size lowers). More than WBFS.
• Tools Used: wit
• Can be reverted? Maybe (same caveats as WBFS).
• Playable on Hardware? No
• Playable on Emulators? Yes

A scrubbed and compressed format designed for maximum space savings without cutting corners on corruption detection, but it's not directly playable or editable. Never really caught on and may be considered de facto deprecated by NKit.

NKit formats - GC/Wii

• Archive-quality dump? Not directly, can be restored with NKit
• Gain: Immediate - (Wii) only for GCZ format.
• Tools Used: NKit.
• Can be reverted? Yes.
• Playable on Hardware? (GC) Yes - ISO-based variant only (Wii) No
• Playable on Emulators? Yes - Dolphin

Apart from its previously mentioned unscrubbing/untrimming features mentioned in the above formats' descriptions, NKit is able to convert any ISO (clean dump or otherwise) to and from an intermediate format applying various reversible changes that optimize the image for lossless compression, including full decryption and optionally splitting the often non-unique update partitions to separate files.

NKit-GCZ (as well as plain GCZ) is less space efficient than NKit-ISO compressed with a powerful general purpose archiver (7zip), however it is directly playable in Dolphin.

But using it for emulation has problems, such as slower emulated load times, breaking netplay if load times aren't the same, and crashing a few games.

Compressing NKit.ISO with a strong compression format gives the smallest size while being reversible.

Wii U

All uncompressed WUD dumps are 23.3GB, which makes compression needed de facto. Some examples:

• Super Mario World 3D: 23.3GB > 2.61GB (88% gain)
• New Super Mario Bros. U: 23.3GB > 1.8GB (92% gain)
• Donkey Kong Country Tropical Freeze: 23.3GB > 11.7GB (49% gain)

WUX (Wii U Compressed Image Dump Archive)

• Archive-quality dump? Yes (lossless, doesn't actually alter the data).
• Gain: Immediate (smaller file than WUD): Around 50% gain usually (depending on the game)
• Tools Used: wud tool. To use, drag-and-drop the ISO on the executable.
• Can be reverted? Yes. The wud tool includes a decompression utility, which recreates the original file.
• Playable on Hardware? No (can be converted to an installable format using wud2app)
• Playable on Emulators? Yes - Future releases of Cemu (and potentially other Wii U emulators)

By Exzap, Cemu's author (release thread). The tool detects duplicate sectors and only stores them once: all the empty ranges end up removed, storing only ranges which contain file or filesystem data. Of course, the original ISO can be reconstituted.

APP (aka NUS/Installable Format)

• Archive-quality dump? No in regard to discs; Partially in regard to CDN data.
• Gain: Immediate (Total size decreases, compared to disc image)
• Tools Used: wud2app (disc image to APP), disc2app (original disc to APP), NUSPacker (extracted format to APP), Teconmoon's WiiVC Injector (Wii/Gamecube disc image to Enhanced Vwii APP), ...
• Can be reverted? No
• Playable on Hardware? Yes (after installation)
• Playable on Emulators? No

Consisting of a folder containing *.app, *.h3, title.cert, title.tik, and title.tmd files, this official Nintendo format is how tiles are stored on discs and some system titles are stored on the console and is very similar to how titles are stored on the CDN (on the CDN the files are named differently, .app files are encrypted with the title key and common keys and the tmd has a certificate chain on the end that is same for every tmd). This is equivalent to the files contained within a 3DS CIA file; however unlike a 3DS title, converting a disc title to digital does not require modifying the title itself, thus leaving the title's signatures valid.

RPX/RPL (aka Installed/Extracted/Loadiine Format)

• Archive-quality dump? No (dump is extracted to individual files)
• Gain: Immediate (Total size decreases). Can be less than a WUX depending on the game.
• Tools Used: see below
• Can be reverted? No, any padding/file order/signature information is lost.
• Playable on Hardware? Yes, use Loadiine GX2
• Playable on Emulators? Yes

Titles installed to USB or MLC are stored in this format, comprised of three "code", "content", and "meta" folders.

Can be extracted from disc images with DiscU, UWizard (follow this guide for usage information) or JWUDTool, and from the installable format using JNUSTool. Can be converted to installable format with NUSPacker.

Ideal format for game modding, less so for playing on console due to Loadiine's intrinsic modus operandi (appearing to the OS as the host title, with potentially different permissions) leading to poor compatibility.

Switch

See overview of formats here.

NSZ

• Archive-quality dump? Yes
• Gain: Immediate (Total size decreases)
• Tools Used: nsz
• Can be reverted? Yes
• Playable on Hardware? No
• Playable on Emulators? No

Game Boy Advance / Nintendo DS / Nintendo 3DS

Trimming

• Archive-quality dump? No (data removed)
• Gain: Immediate (ROM dump size lowers).
• Tools Used: NDSTokyoTrim (GBA/DS/3DS), rom_tool (3DS), GodMode9 (can directly dump DS/3DS cards in trimmed format)
  NDSTokyoTrim: Drag-and-drop roms, and press "Trim". The original file will be overwritten!
• Can be reverted? No. Some GBA/DS games may be broken by meaningful data at the end of the ROM being mistaken for padding.
• Playable on Hardware? Yes, same file format.
• Playable on Emulators? Yes

Trimming involves deleting bytes from the end of the ROM up to until the first different one.

However, with no mandatory allocation table, it's not intrinsecally possible to identify the true end of the used area, and therefore some GBA/DS games can be broken by naive automated trimming (for example Golden Sun DS).

CIA (CTR Importable Archive, 3DS and DSiWare only)

• Archive-quality dump? No in regard to gamecard dumps; Partially in regard to CDN data.
• Gain: Immediate (smaller than original ROM - not applicable to digital titles).
• Tools Used: GodMode9 (can directly dump 3DS cards or installed titles to CIA, as well as converting 3DS to CIA), 3dsconv or 3DS Simple CIA Converter 5.0+ (3DS rom to CIA); makerom (3DS to/from CIA, NCCHs/DSiWare/ELF+RSF to 3DS/CIA), make_cia (DSiWare to CIA)
• Can be reverted? No, if starting from a gamecard dump (update partitions removed, main content's ExHeader modified to change the media type from CARD/NAND to SD Application)
• Playable on Hardware? Yes, after installation.
• Playable on Emulators? Yes?

Official Nintendo format for developers to install digital titles, later became the most popular format for installable homebrew and game backups owing to the relatively low marketshare of flashcards and emulators (and, for the former, strong impopularity) in favor of CFW.

Equivalent of the Wii's WAD (for digital titles only) and the DSi's TAD (again for digital titles only, and with basically no popular support due to the lack of any homebrew title installers supporting the format - not to be confused with an homonymous unofficial format nor with the files produced by the official export-DSiWare-to-SD feature)

NCCHs (CXIs and CFAs, 3DS only)

• Archive-quality dump? No (partition table/ticket/signatures... removed), compared to CCI/3DS and CIA
• Gain: Immediate (smaller than original ROM - not applicable to digital titles).
• Tools Used: GodMode9, ctrtool, ...
• Can be reverted? No (result will be unsigned)
• Playable on Hardware? Not really, unless packaged back into a CIA or 3DS.
• Playable on Emulators? Yes

3DS and CIA files, as well as already installed digital titles, are comprised of one or more NCCHs (also called "partitions" or "contents"): the main content (number 0) can either be an executable CXI for software, or a non-executable CFA for a data title. Additional CFAs may be present, with conventional index numbers. Citra can run CXIs directly.

Xbox 360

It's certainly better than keeping 8.5GB images, but the conversion is too substantial and irreversible to be suitable for archival purposes since it affects data structure tables as well.

XEX (Xbox Executable) + Data folder (a.k.a Spilling The ISO Guts)

• Archive-quality dump? No (dump is collapsed to individual files)
• Gain: Immediate (total file size decreases). It still works with Xenia
• Tools Used: XBOX 360 ISO Extract, Exiso-GUI or Exiso. (These might be useful for rom-hacking too I guess?)
• Can be reverted? ISO could be rebuilt, though not accurately.
• Playable on Hardware? Yes. Using a JTAG or RGH exploited console.
• Playable on Emulators? Yes - Xenia.

Rebuilt ISO

• Archive-quality dump? No (missing data)
• Gain: Immediate (total file size decreases). It still works with Xenia.
• Tools Used: ISO2GOD (also a "Games on Demand" X360 image converter).
  Under Settings, set the output and rebuild path to the same location. Check "Always save rebuilt ISO" and set Padding to "Full (ISO Rebuild)", then save changes.
  "Add ISO", and press "Convert". Keep generated ISO, and delete generated folder.
• Can be reverted? The padding quantity information alongside the original data structure is lost forever.
• Playable on Hardware? Yes. If the rebuilding process didn't damage anything vital.
• Playable on Emulators? Yes - Xenia.

Other Tricks

Storage Tricks

These methods have the advantage of being compatible with EVERY emulator, even those without proper support for compressed ISO/archive formats.

NTFS Compression

You can enable filesystem-level compression (like "NTFS Compression" in Windows) for the directory containing your ISOs/ROMs. This has a very noticeable space gain and doesn't affect the emulator's functionality. It's surprisingly more effective than many people would like to give this credit.

• Archive-quality dump? Yes

Decompression on Demand

You can keep your ROMs/ISOs compressed in a 7zip archive and use RocketLauncher or any other Frontends to decompress 7zip archives and pass the contents onto the emulator.

• Archive-quality dump? Yes

FileSystem Deduplication, alternative file systems

If you use macOS, you can look up for fuse filesystems which support compression/deduplication.
If you use windows, you can fire a VM/store your games on ,a pc with a transparent network file server running linux/bsd.(might be viable on low latency networks, and a fast file server)
On linux and bsds one can use, filesystems with de-duplication(and possibly transparent file compression) like:

• Squashfs
• Note: you can mount the filesystem and use an overlay file system to add a new file, and then create a new squashfs against an overlay folder.
  • ReadWriteable? No read-only.
  • Deduplication? Yes, block level.
  • Active Deduplication? No, Deduplication is triggered by the user, during filesystem creation.
  • Available compressors: zstd, xz(lzma2), lzma, gzip, lz4, lzo.
  • Compression level control: Depends on the compression algorithm, xz dictionary size depends on filesystem block size.
  • Transparent? On Linux and BSD via kernel. Squashfuse avaialble for macOS.

• Btrfs
  • ReadWriteable? Yes.
  • Deduplication? Yes, file and block level.
  • Active Deduplication? No, Deduplication is triggered by the user.
  • Available compressors: zstd, zlib, lzo.
  • Transparent? On Linux via kernel.

• Zfs
  • ReadWriteable? Yes.
  • Deduplication? Yes, file and block level.
  • Active Deduplication? Yes, deduplication happens during writes.(high memory requirements)
  • Available compressors: zstd, zlib, lzo.
  • Transparent? On Linux, BSD, via kernel. Openzfs on mac os(?).

• Xfs
• Note: Without de-duplication it is already quiet efficient at storing files.
  • ReadWriteable? Yes.
  • Deduplication? Yes, file level via duperemove.
  • Active Deduplication? No, deduplication is triggered by the user.
  • Available compressors: None.
  • Transparent? On Linux and BSD via kernel.

• Archive-quality dump? Yes

Destructive Modification

Sometimes, the most bloated part of the game is the game itself, either intentional or because of poor design choices. For instance, many PSP JRPGs have a specific FMV movie duplicated a dozen times, and the Megaman Collection on GCN stores its sound data using an uncompressed format bringing the size of that portion alone to 1GB. In many of these cases, there's just not much you can do about it without destructively altering the game's data.

Note, "deleting" often means replacing the file with a 1KB dummy file to prevent the ISO file structure from collapsing on itself, but sometimes such care isn't even put into ensuring it's still in a functional state. Some examples:

• delete all videos: this brings Super Smash Bros Brawl to 4.7GB (mainly due to Subspace Emissary).
• delete all voice acting and occasionally sound and music: this brings Xenoblade PAL to 4.2 GB with even only one of both dubs removed.
• delete unused content you could get with emulator cheats like rooms and stuff.
• delete "extra" languages other than the language you need.
• If a game has multiple quests, then you could try deleting characters/side-quest you don't like as much.

Tools used for this are modding tools for rebuilding file trees, like UMDGen (PSP), Tinke (DS) and also regular ISO tools (PS1, Saturn).

Since this results, in most cases, in very noticeable detrimental effects in gameplay (if the game doesn't crash outright), this is nothing short of mutilating the game image. Sadly enough, some of these dumps make it to sharing sites.

Avoid resorting to destructive modifications since it can lead to random crashes and unexpected behavior, especially in games with lots of shared assets. [Verify your dumps] to make sure you don't have these, and if you still want to reduce size, prefer using other methods or uses decompression on demand. One reason to use a destructively modified dump is for burning your own Dreamcast games as GD-ROMs were over a gigabyte in size and CD-Rs top out at 850MB.

• Archive-quality dump? No (Removes data)