The witcher 2 had 16gb and about 39 hours of gameplay
I find it weird that Rage have 21 gb, judging by its characteristics
RAGE's engine uses a new kind of texture technology, so the textures take up a much larger amount of disk space.
Skyrim does not use the id tech 5 engine, so this is not a concern.
Also, comparing Skyrim to the Witcher 2, Skyrim will not have very many cut scenes (or perhaps no cut scenes at all). In comparison, Witcher 2 has a lot of pre-rendered cut scenes that take up more disk space.
Hours of content =/= Increased File Size
Quests don't take up very much disk space at all, especially when they don't use pre rendered cut scenes..