Some games respond to compression better than others. Titles with repetitive data, large audio files, or inefficiently packaged assets tend to shrink the most.
: Because it is one large file, it avoids the performance hits caused by file fragmentation common in folder-based games. The Reality of "Highly Compressed" PS3 Games
Tools like RPCS3 (emulator) and webMAN MOD (on physical hardware) are designed to read standard formats. Highly compressed/modified files often cause crashes, infinite loading screens, or "black screen" errors.