It's rather obvious when you sit the F650GS and the F800GS next to each other that one has a much more aggressive look, I guess you could call it masculine. The side panels with the big air intakes, the beak, the twin discs, the bigger spoked wheels and the thicker USD fork legs etc all contribute to that. I personally think the F650GS looks pretty ugly stock, that's why I modded mine.
On the new 2013s the difference is much less noticeable, and the F700GS is improved a lot. Still prefer the old F800GS bodywork over the newer models, though.