In standard computing, compression comes in two flavors: Lossless (like ZIP or RAR) and Lossy (like MP3 or JPEG).
When you see a PS2 game shrunken from 4GB down to 90MB, you are not looking at standard compression. You are looking at "Ultra-Lossy" repacking. Here is what has been stripped out to achieve that microscopic size: Highly Compressed Ps2 Games Under 100mb
The result is a "proof of concept" more than a premium experience. In standard computing, compression comes in two flavors:
If you see a claim of “PS2 Game under 100MB,” manage expectations: The result is a "proof of concept" more
So you have downloaded a file named Game_Name_100MB.cso. How do you run it?