If you include all the islands that you could actually go to (mainly by swimming to them), the difference is not all that great. Did Oblivion feel any bigger? Not to me.
Skyrim is going to be pretty much the exact size of Oblivion . . . yet Skyrim has 8 different regions . . . so each region is going to average only 2 square miles . . . 2 square miles of tundra is hardly a "huge area."
Plus both Morrowind and Oblivion included mountains that you could climb . . . some of you guys act like they were just flat land masses.
Well, even with the islands and all of that, Morrowind was more or less 2/3 of Oblivion, and there was lots of places that were not accesible except by Levitations. Did it feel bigger? Yes, and maybe it was the neccessity of looking for a path to go through the terrain what made it like that. I think we are going to see this back.
Obviously, previous games had verticallity as well (Cyrodiil was a valley), but not to the extent Skyrim will have, I suppose.