Use 7-Zip (PC) or ZArchiver (Mobile).
: Originally popular for the PSP, it is also supported by some PS2 emulators. Advanced "Ripkit" Techniques Ps2 Iso Highly Compressed
Finding the ISO is only half the battle. You need software that can read compressed formats. Here are the top emulators that work seamlessly with files. Use 7-Zip (PC) or ZArchiver (Mobile)
Start with one game: compress it to CSO, test it in PCSX2, and see the difference. test it in PCSX2