That's assuming they used a full 50GB disc. We used to switch out between as many as four CD's in the PSX days. However, to be able to render all of the various areas in the game at any point during a game would require the necessary information be on the discs, which would mean each disc containing a lot of the content (HD textures, audio, etc.) that's already on the other discs. I suppose looking at it that way, it could take more than the equivalent disc size in DVD's to do the job.