I would also like to point out that even in the event of the overworld being as small as you're thinking, in retrospect it shouldn't matter because what's important in the end is a maximized
experience, something which a huge overworld may
contribute to as your thinking, yet is not necessarily
required to produce. In actuality, the immensity of the overworld that we seek is really done in the subconscious hope of it optimizing our experience; it is therefore the immensity of the
experience that is what we are actually looking for. Case in point, neither
Majora's Mask nor
Ocarina of Time had enormous overworlds, yet they are remembered for being enormously successful entries in the franchise due to the impact of their experiences. Seeing the reviews being released so far for
Skyward Sword, I would say you can expect an
experience at
least equally as immense, if not more, as
their FULL hands-on impressions boasting the game's magnitude are far more accurate to what we may be expecting from it at this point than our speculations can be, limited as we are to the selective information released thus far. More simply: they've played the full game, we haven't; they say the game has enormous magnitude, they have a lot more ground under such a claim than any opposing notion we might be entertaining in our heads. There
is credibility in trusting what they're saying right now, even despite any chances of developing a conflicting personal opinion when you actually play the game for yourself.
Regardless, I think there's too much worry going on about this right now. Just stop worrying and trust the game to deliver to you how it will.