Well, I guess that depends on your game. How far in are you? And believe me, there are WAY more side quests to do then you imagine, they just aren't obvious. Numerous times I've run into little quests that started out as little miscellaneous 'go check this place out'.
One my second play I haven't even done the MQ yet and I'm still finding quests to do. You just have to explore.
Unless, of course, you've already explored every location and beaten the MQ.... then yes, you are left with radiant quests.
As for the map, I believe, from my reading, that the map is actually larger than Skyrim, but appears smaller because there are no mountains or large impediments to your travel. Skyrim felt bigger because some places you had to take a particular path to get there and you were dealing with different environments. In the commonwealth, you can pretty much take off in a direction and reach the other side of the map if you want. Couple that with the landscape being the same throughout and you get a small world feeling.