Interesting explanations!
I thought earth ponies need the largest hooves because they do more physical work like farming, and unicorns should have the smallest because they do not have to use hooves to hold or operate things to work.
I don't think unicorns need sturdiness for using powerful magic because there is about only one in a thousand unicorns (if not less) with powerful magic (there is none in G5, and there is only Twilight, Starlight, and Star Swirl in G4).