Older "ripkits" can be used to manually remove "dummy files"—empty data added by developers to fill the DVD—which can sometimes shrink a multi-gigabyte game to a few hundred megabytes. Best Emulators for Small Games
Popular for PCSX2, converting an ISO to a .gz file using 7-Zip (Ultra level) can save significant space while remaining playable. highly compressed ps2 games under 200mb install
If you have larger ISO files, you can compress them yourself using specific tools to save space without losing gameplay quality: GZIP Compression PCSX2 emulator supports loading games in the format. Using Older "ripkits" can be used to manually remove