id have big, big plans for Rage! Or, they did have big, big plans. Plans they've had to cut back on so the game can be squeezed onto the 360. id's Tim Willits explains:

The PC is limitless in the amount of data you can put on it. The PS3 has about 25GB. But the Xbox 360 roughly has 6 to 8 GB of data. We're hoping we can squeeze the game down to two discs for the 360 version.

As an example, Willits says that the game was originally meant to feature "several" wasteland areas. But because they wouldn't all fit on the 360, they had to cut the number of wastelands (on all versions) down to just two.

Rage Held Back By Xbox 360's Limits [1UP]

UPDATE - id's Tim Willits has issued a statement strongly denying 1UP's report that any content has been cut from the game. Rather than wastelands being removed from the game, he says they went from having 5-6 smaller areas to two large ones, so that "for the 360...it would play better with one large wasteland on each disc so there would be no loading between wastelands".