(The Big One) CHD compresses data without losing a single bit. On average, a PS2 CHD is 30-40% smaller than the raw ISO.
The definitive versions of these action-RPGs are still the originals for many fans.
Most users prefer to convert their own legally owned physical discs using tools like (part of the MAME project). This ensures you have a clean, private "exclusive" copy of your game that is optimized for modern emulators.