The Guide to Highly Compressed PS2 ISOs: Maximize Your Collection
Primarily used for PSP but supported by some PS2 emulators. It provides similar benefits to CHD but is generally less efficient. Zstandard (zstd): Some emulators like PCSX2 now support zstd compression
Today, the community has moved away from the "permanent" lossy compression of the past toward "transparent" formats:
BIOS Files: Regardless of compression, you still need a valid PS2 BIOS to boot your games.
You would download a 50MB file, start the extraction, and your family computer would essentially become a space heater for the next 12 hours. You’d go to school, come back, and find the progress bar at 84%. If your power flickered for a millisecond, the entire process was ruined. But when it worked, that 50MB file would bloom into a full 4GB ISO like a dehydrated sponge hitting water. The Modern Standard: CSO and ZSO