TrueTGN

Plain and simple, my 360 game dvd's may only hold 8 or so gigs of space. BUT! Developers can compress a lot of information to a disc of that size, allowing you to have a lot more space then you think is available. Im not sure on how well they can compress them now, but my guess is that they did some nice work with Oblivion and made it fit nicely on one disc. (Which they could and probably will do with a lot of other games if needed)