The mountains will create an illusion that the world map is bigger, just like Morrowind which was significantly smaller in world space then oblivion. Oblivion's problem was people could run in a straight line unstopped, or see the whole map from a high spot which made the world seem small. Also you could say slow running speed also made the world seem bigger.
The dungeons in Oblivion on the other hand, believe it or not, was crafted by ONE guy
. They've hired a few more people to work dungeons since then, so it should be more varied. :spotted owl:
This, in one of the pics you can clearly see all these path on the Mountains, in Cyrodill they were only mostly on the borders so they sevre as walls, while Skyrim will have this too, there will also be stuff on the mountains to go through and all kinds of twisty paths.
Water can also make a world seem larger come to think of it, because of the big lake in Cyrodill you have to walk around it(though I guess you could swim). Hmmm.
As for the dugeons in OB, that is amazing that they had one dude do like 230 dungeons, no wonder they seem alike. SR has 8 doing about 120.