A standard PSX ROM (usually in .bin/.cue or .chd format) is a 1:1 copy of the original CD-ROM. A "highly compressed" ROM is not simply a ZIP file. It is a file that has undergone algorithmic reduction, often using formats like PBP (PSP Eboot) or CHD (Compressed Hunks of Data).
True "high compression" can reduce game sizes by 60% to 85% depending on the game's content (FMV-heavy games compress less; games with lots of code compress more). Psx Highly Compressed Roms
Downloading pre-compressed ROMs from random sites is risky (pop-ups, malicious ads, fake EXE files). The safest method is to compress your own from verified Redump.org disc images. A standard PSX ROM (usually in
Cause: Bad compression or missing sbi subchannel files (for anti-mod games like Libble Rabble).
Fix: Re-rip the disc using CloneCD (which captures subchannel data) before compression. True "high compression" can reduce game sizes by
While less common for PSX specifically (more common for PSP), you may still see this around. It is a generic compressed ISO format.
Some games are legendary for their high compression ratios. Here is what you can expect (original size -> compressed size):