Yeah, I haven't read much about Skyrim, but I clearly remember one of the interviews/articles stating that cities will have their own worldspaces.
I'm kinda ambivalent on the issue, actually. Separate worldspaced does mean they can make the cities larger on the inside without taking up too much space on the "true" world map. Towns in games are always so pitifully small. A smithy, two inns and one general store in the whole capital? Quite the metropole indeed.
So yeah, that's a break from "realism" I would willingly make to have cities that actually look something like large settlements, if not quite town-like.
That ain't the reason for seperate maps at all. Oblivion's cities were the same size on the inside as they were on the outside. They do this for performance reasons.
Never said it was "the" reason for it, in Oblivion, Skyrim or any other game - I know it isn't. The point was that that could be a positive side of having separate world spaces, it they want it too. In my eyes, there's little reason to having them the same size if you won't be able to travel seamlessly between them anyway.
I never did buy this "new engine" baloney.
From what I get out from reading between the lines (and it might just be my negativity showing itself, mind you) it's technically a new engine, but it works much like the old one. A more modern, re-made version of it, you could say.