Expansions are no where near the size of the actual game.
Expansions/DLC usually run on the exact same engine.
New Vegas is bigger or the same size as Fallout 3.
It uses a tweaked engine. Meaning, it wouldn't run on the same engine as Fallout 3. Yes, they are almost the same but again, it was tweaked and wouldn't be compatible with each other.
It is not a direct sequel. They could have easily called this Fallout 4 if they wanted to.
Would you call Vice City or San Andreas expansions? Hell no you wouldn't. Those offered more than GTA 3 did and used a slightly modified engine. Although SA had improved graphics. And guess what? Those weren't sequels. They were side games. Still canon though.
Please learn the difference between expansions/downloadable content and actual games.
If you were to download this game it would be as big as Fallout 3 or bigger in terms of size. I wouldn't be surprised if this game was at least a gig bigger than Fallout 3 (which is about 5.5 according to my install). But size doesn't really matter. It's all about the content. And like I said, New Vegas is at least the same size as Fallout 3. I heard it's bigger.
If you guys think it's an expansion, don't buy it then. Pretty simple. Wait for a price drop or whatever. A full game doesn't need to look drastically different or anything. Some games use the same engine or a modified engine and the developers focus obviously wasn't the visuals.