If all five cities are as massive as they say and as massive as they appear in the trailer and with the massive mountains scattered around how can this really be "roughly" the same size? Don't forget there is supposedly over twenty towns/settlements that are decent size as well so this just doesn't make sense.
If you look at the official map of Tamriel then it shows Skyrim as being about two thirds of the land mass of Cyrodil if they were both flat, however as Skyrim is mountainous it has alot of land area that is going to consist of steep hills and slopes. These slopes are going to take up less area in a two dimensional map but more land area three dimensionally. Even counting that to make it roughly the same size I think they're having more large settlements than Oblivion had.
I think what is going to happen is that the area is going to be less landmass but with the mountains containing things on their slopes and acting as natural barriers to force travel around them rather than enabling more or less direct paths everywhere, it will be made to feel roughly the same size as Oblivion. They might also have more extensive internal areas than Oblivion that are instanced from outside world, at least I'm lead to believe they are instanced as they were in Oblivion and Morrowind. Roughly the same size could also mean, less area but more stuff within that area.