According to the official map.Solitude = Imperial CapitalWindhelm = Rebel CapitalRemember, there is a civil war going on. There's going to be two capitals.
I find that odd, (though I too saw the map) since it said in one magazine that Windhelm was where all the Imperial guards hang out.
Well if we have five "massive cites" than the capital has to be even bigger. Maybe 4-5 times of the IC?
WHAT? What makes you think that the capital city is not one of the "massive" cities. "Massive" means "really big", not "big, but other things in the same area are bigger".
I also doubt we will have cities with 400-500 buildings. Very much. There's just no way they have enough resources.
EDIT: I think my post sounded a little flamish. I didn't intend it that way if you got that impression. But just to put it in perspective, that one major city you are proposing would be about the same amount of buildings as every city in Oblivion combined. Yes, they have double the resources, but still, it seems improbable when you take into account that there are supposedly 5 massive cities and 20 smaller settlements containing at least 10 buildings. And the fact that the map is roughly the size of Oblivion.