Uh yeah but whats the biggest city in Skyrim, - Markath? There are about 85 NPCs in Markath including 2 named dogs, add to that some generic guards.
And yes you can go in every door. There are 23 visitable areas. In the game's biggest city. And that's including the mines, ruins, the hall of the dead and some open air locations like the marketplace.
Now I don't know about you, but I vastly prefer W3's approach with hundreds of generic NPC's and tons of buildings (no I haven't counted them), un-enterable as they may be. To me its a much more convincing representation of a city, as when you're visiting such a place on business you won't be interacting with the majority of the residents, nor barging into their homes.
Sure it would be NICE to be able to do so, but in this case I'm satisfied with the backdrop of appropriate scale, rather than the weird double-think of Skyrim where a crowded city has the population of a backwater town.