He doesn't know what he's talking about. Skyrim will be like 12 to 15 GB, more than any single DVD can hold.
How can you claim to know more than him?? Unless you're partial to some sort of industrial espionage the rest of us haven't been informed of, then you can't say that for certain. Judging by the fact the heightmap will be the same sort of size as Oblivion (we have been told this), thus unless they make a crazy amount of high poly meshes and high res textures, there's no way they could reach 12 GB. And seeing as it's still the same hardware (albeit improved engine this time round), it won't be massively massively bigger than Oblivion. And if worse comes to worse then they'll just give the 360 even lower res textures.
EDIT:
@thatkhajiitbock, that's just nonsense now. Not debating your points about MGS (because I don't know either way), but more space means better textures and in the case of TES, a larger game world. My Oblivion folder on PC, complete with all of my mods, is 26GB. And I've seen bigger folders. You can't tell me all of that is redundant, and if you try then you clearly have no idea