It's actually bigger. I think it's the emptiness that made the FO3 map to seem bigger.
It does actually make it seem bigger, oddly that's why I like the emptiness. Having more empty space, where there is more of a travel between places, just makes it seem bigger even if there is just less to see. And it's a wasteland, there should be a lot of empty space.
As far as just the number of squares though, I don't think it's that much bigger, if at all. Map size it seems, for whatever reason, has hit a sealing with a lot of games, and FO3, NV, etc. seem to keep it around the max. Which is really frustrating, because I've been waiting for a game that really increases the map size to something impossible to miss. And again, it doesn't even have to be jam packed with stuff, just spreading stuff out so there are more empty areas and more travel time between major areas would be nice.