They need to keep to lore and the maps or there is no need for neither.
But Oblivion
isn't to scale with real life. It's a tiny scale model of what Cyrodiil is supposed to be. Morrowind is, in fact, larger in the sense of it being to a more proportional scale to real life (and even it's tiny to how it should be), because it's a much smaller landmass than Cyrodiil is, and yet it's, by proportion, larger. And BOTH are way off when you compare them to Illiac Bay in Daggerfall, which is a 1:1 representation, size-wise. And Daggerfall's so old we can't base the procedural technologies of fifteen years ago with how Skyrim would turn out if it's a larger game.
It will be larger than Oblivion was, because that's what people expect, and because it makes sense for Bethesda to push closer to a real-life scale as technology advances. With Daggerfall, Bethesda bit off more than they can chew. They've quite a larger number of games under their belt now, and that simply will not be a problem.