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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.
The problem that I have with this assertion is that big widgets works fine for really simple applications as browsers, e-mail clients, RSS readers and such.
Throw something more complicated like a 3D modeller, non linear video editor, CAD program or something like that, which often has to show several options to the user at the same time for the sake of productivity and you´ll run out of luck. It simply doesn´t work for these types of applications.
GNOME has been leading this annoying trend on OSS of oversimplification of everything, even when it comes in the way of the original purpose of a said program as if requiring a user to learn to use something was a capital sin. I don´t mind a browser simple to use but I can´t see big icons working on Blender, for starters (And no... Blender has NOT a poor UI nor is this an OSS flaw. Any 3D application worth its salt has a daunting UI, like Lightwave 3D or Maya, and (god forbid!) actually require their users to learn something before start to use them.)
I´d appreciate even more GNOME developers efforts to simplify the interface as long as they kept the possibility to power users to take full advantage of the software, allowing them to adjust the software as they see fit after learning. The problem here is that the user can outgrow the limits imposed by the the GNOME DE in a very short timeframe and after that they´re screwed. I, for one, feel more constrained within GNOME than on Windows.
That alone, besides the eye candy factor, makes me prefer KDE even if I do know that it could use a little bit HIG on its default state. The power to make it suit my workflow and my preferences for the price of 30 minutes tweaking it is more than worth it.