That is a really cool and informative map comparison. Its really interesting to see how small GTA is even though that world is pretty huge.

To answer the OP question,"Can a game be too big?" my answer is , Yes... in theory.

Although I havent met a game where bigger isn't better, and I want even Bigger... Yess gigantic even.

Elder Scrolls II was HUGE

is F:NV anywhere near as big as Just Cause 2?

I actually think current games are too small. Just Cause 2 and Daggerfall were steps in the right direction!

I think that the size of the game makes it harder to polish, but if there could be the time I bet that the game could have had less bugs. I am at 80 hours into the game and I still haven't gone properly into Primm to follow that storyline. I don't care if I still haven't reached the Strip yet (stupid Shane), I am having a blast and making a fortune (400,000 caps currently) off an exploit I found playing Caravan. :D

I think the map is slightly overloaded with locations.

No. 

Nah maybe as big as Far Cry 2

No, more content the better. People nowadays are suckers for content. People want games that last them a long time. The problem is if the content will cause game breaking problems.