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It's a design decision. Personally I prefer the relatively larger sizing of GNOME, but arguing about that is pretty silly considering that it almost entirely depends on screen resolution. The solution is to make the interface entirely scalable (like fonts already are), then we can argue about the most tasteful relative sizes. Cairo is one step into this direction.
In any case you can't say smaller = better, it's always a tradeoff. For example, larger buttons are faster to click on and thus more efficient (Fitt's law). It would be a mistake to think that advanced users aren't affected by this, just because they are good at mouse control.