It's worth mentioning that New Vegas used its locations for a lot more. Almost every single one was related to a quest or something, if not several, and even if individually some places were smaller, it was made up for in actual gameplay content. Meanwhile Fallout 3 had tons of places with one single NPC to talk to, a bunch of dungeons of which only a handful had any quests (journal or non-journal), etc. Many of them were just a single burned-out building with nothing in it but generic mobs, or cave dungeons with nothing in them but monsters, etc., and it seems like they were included just so the player would have some sort of fast travel point.
It was hardly a small game by any means, and I thought some areas were well done (at least visually), but I'd say in terms of actual gameplay content New Vegas probably has twice as many. It took me something ridiculous like 80-100 hours to complete every quest I could in New Vegas, Fallout 3 I think I finished in about 35-40 at most... sure you can get more out of it if you explore every single possible location, make sure you've got every random encounter, etc.
I do think one thing Fallout 3 might have going for it is that after the tutorial, it feels like a much bigger, open world. New Vegas had more artificial barriers and walls, the desert was more corridor-like in the early parts, and it felt much more linear and guided early on just in terms of where you could safely travel. I can see why some would feel the game is smaller, especially if they just rushed through to Vegas. There are so many quests to find and random NPCs to talk to that are so easy to skip by and miss if you aren't paying attention, whereas Fallout 3 shoved most of its content in players' faces (i.e. enter an area, NEW QUEST), that I can understand why it'd feel like a bigger game to some.
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