The Xbox Disc holds up to 10gigs, its not limiting anything so don't even go there.
Bethesda said its the new engines compression that handles data way better, thats why its small, and it still has way more content then previous games.
Unless im mistaken, Uncharted 3 has over 50gb of data.
So, by that measure, the Xbox disc is VERY limiting.
I know BGS have great compression methods for Skyrim, but If the game developers had a 50gb margin to work with, the detail and amount of content in the game would be far, far superior to anything we will get from Skyrim (not to say it wont look great, and be great to play).