Actually, i think that New Vegas & Fallout 3 had dealt with the space/population in a very opposite way.
- In Fallout 3, the actual city of Washington is pretty big, with a lot of locations to explore. Also, the city is conveniently in the corner of the map, so it implies that you only see a tiny part of the city. On the other hand, the game want you to believe that there are some travelers, caravans, patrols, (and 3dogs informants that run to him everytime you do something) while you barely meet someone outside. The settlement are scarce, tiny and doesn't seem to involve a logical way to replenish their food, water, children, and ressources. So, while the space is believable, the population isn't.
- In Fallout New Vegas, the actual city that is on the center of the map doesn't feel like a big city, or even a city. You can circumnavigate the whole city in less than an hour, the buildings are very low sized, and there are no landmark. It seems like a small or medium village that have some ressources that interests some factions, but not the biggest city in Nevada. The game would have been much more believable is it wasn't named New Vegas. Then, there are also Nipton, Goodsprings, Novac, that, according to external sources, are supposed to be cities on their own. Ingame, they feel like they are on the boundaries of Vegas, the first step of the travelers, that are hurry to sleep before reaching the Strip or Freeside. On the other hand, the way the Mojave is populated make sense. There are a whole lot of people in every settlement, factions headquarters, or even in the roads, with caravans (far more than Fallout 3), travelers (voiced by Mike Novick), factions patrols. Unfortunatly, most of them are generic, but they are here, and help feel like that the area is, in the same time, fairly populated with some hotspots that are constantly watched, AND also have areas where you get ends up alone, trapped and killed without anyone noticing. So while New Vegas deeply fail to show places that are supposed to be big, i think they succeed in making the area fairly populated.