Actually, it's a pacing question.
No. They have to fit all the engine code on every disc, as well as all the relevant assets for that disc's content. When you get airships and crap, at any moment you can potentially be going to ANY city, so as you go along, the number of assets you need per disc -- and as such, the amount of space being taken up per disc -- goes up. You start hitting limits. Thus, you need to close off earlier areas (BLOW UP ALEXANDRIA/THE WORLD/WHATEVER) or just spread out later areas.
At least that's the official word. To me, I don't see how town assets could really be taking up that much space, unless your programming team consists of idiots. Then again, this IS Square we're talking about, though, so...
I don't buy your explanation, though, because getting an airship really doesn't change the scale of how you're viewing the world at all. You just move faster.