If you are trying to save space for use with the PCSX2 emulator , there are legitimate ways to compress your own legally obtained ISOs:
The God of War Collection on PS3 runs at 1080p, 60 FPS, with trophies. You can emulate this via (PS3 emulator), but it requires a powerful gaming PC. God Of War 2 Ps2 Iso Highly Compressed
Locate your downloaded highly compressed file (usually ending in .7z or .rar ). Right-click the file and select using 7-Zip. If you are trying to save space for