I wouldn't assume Skyrim is much bigger than Morrowind and Oblivion.
I wouldn't either. I also wouldn't assume that it isn't. Bethesda tends to majorly rework things with their games between iterations, and in this case they've created a new engine specifically for the new game. It's entirely possible that wanting to provide a significantly larger landmass was part of the decision for creating that engine. Also possible that it wasn't. Again, I don't think anyone really knows enough to say for sure either way.
Oblivion was only slightly bigger than Morrowind and it's sort of the limit of what you can produce at high quality of the time of a games development.
Neither of those statements is really true. Oblivion is nearly double the size of Morrowind (Morrowind is 9 square miles, Oblivion is 16), and you can produce considerably larger and
far higher-quality environments than that (Red Dead Redemption has a map that's supposedly 28 square miles, and the world itself is considerably more organic, tends to have considerably more complex geometry, is far more natural both in design and appearance, and - yes - has interiors). Even Just Cause 2's map is actually significantly more complex and detailed than Oblivion's (the terrain isn't shaped like it was generated from a low-res heightmap, for one), regardless of the lack of interiors.
I'm not entirely sure why so many people think this, but Bethesda most certainly is not near the limit of what can be achieved, even if we limit it to the current higher-end console hardware.
On top of that, the issue you bring up can be adressed in a lot of different ways. If you'd add enough ways to teleport and plenty of travel paths, it shouldn't be a problem no matter what the size is.
Again, not really true. Lots of ways to teleport does absolutely nothing to help any non-magic class (which would consist of... most of them) aside from getting from town to town - there's a reason I specifically used town-to-dungeon travel in my example, since boats generally don't run that way. And more paths don't help at all when those paths are
obscenely long.