Morrowind is 10 square miles
Oblivion is 16 square miles
Skyrim is 14.3 square miles.
The 16 square miles (not including the Shivering Isle expansion, planes of Oblivion, dungeons, etc) appears to be the square miles in the rectangular map.
If you actually count up the accessible area it is more like 9-10 square miles, or the equivalent of a 3x3 mile square. Which is still very large for a game.
Skyrim feels a bit smaller too me. Even though the mountains make it seem larger because there is often less of a straight path than in Oblivion (much like Morrowind). Still Skyrim is big enough for an amazing game.
And I don't know how the OP got those numbers, but Skyrim isn't even remotely 40-50 square kilometers.