Fo3's map feels bigger, but a lot of the locations there are pretty simple, and there is lots of buildings you cannot enter. While in FNV, i think you can enter all of the vanilla buildings? So there really aint no empty props.
But, for size, fo3 would win, I think.
For content, fnv would win, maybe.
I mean, if obsidian had gotten more time from beth I'm sure they would have filled up more of the empty spaces..
If a stupid idea works it's not a stupid idea.