It varies indeed. I've pakkiso'd many PS2 isos lately and some games just can be compressed incredibly well due to huge dummy files on the disc. Metal Slug 4, for example, compresses to about 2% of its uncompressed size. Disc 3 of Metal Gear Solid 3, on the other hand, stays 94% the size of its uncompressed counterpart.
If I were to estimate I'd say that on average PS2 games can be compressed some 40-60% with PakkIso (or plain 7zip, of course).
F1ReB4LL wrote:Incorrect question, IMO. You should ask about the difference in % between zip/rar compression and pakkiso+7z.
Damn, you are right. Every game is compressed in *.rar on WinRAR 3.90 using best level of compression. I guess the % difference will be quite low there.