For a video game the strip is pretty impressive, actually, especially seeing as how it's just what's been restored after a nuclear apocalypse. There are already a ton of quests all over the place. Did you really expect something the size of the real life Vegas strip? It would have taken years just to build that, much less the rest of the game. They could have made it large GTA-style, but the places in GTA have nothing to do in them for the most part...they're just decoration that provides more area to justify the space occupied by the roads.
I don't feel the size of the strip to be underwhelming, in fact, if it were much bigger I think it would be too large. Now the population of the Strip, on the other hand is disappointing. It seems like every casino has more employees than guests (which is not a plausible business model).
Technical limitations. This engine doesn't scale well when you add lots of NPCs. The system requirements would go way up if they added more.