I've had quite a few similarly impressive archives for PS2 games recently. I even had one that was around 600MB compressed, and a full 4.7GB DVD once extracted. Quite amazing how they manage to squeeze the files down so small. Although it probably has more to do with the original game.
Games that already use compression on the original disc to hold more won't compress as well as games that are just filling the game disc with normal uncompressed files.