As I write this post I am downloading Grand Theft Auto: Episodes From Liberty City. It is 16GB, contains two stories based in the same setting as Grand Theft Auto 4 and, in all fairness is absolutely huge. Skyrim is also absolutely huge, but you wouldn't know it to look at the file size.
Some of the Twitterati, experts that they are, have been adamantly refusing to believe that ALL of Skyrim could fit into a 6GB packet, Pete Hines took to his account to explain the situation.
"Oblivion was, what, 4 (Gb) something?" he argued. "Did it have more content than these games being mentioned that have 2x or 3x the install?" Which is a fair point. It's difficult to argue the amount of content is lacking without access to the game; the majority of people complaining will, of course, not have had access to the game.
But how can it be done?
"New engine and we’re much better at compression (art/voice/data). It’s huge yet more optimized than we’ve had before. And faster."
Don't worry, Skyrim will still keep you going till Christmas and beyond.