Much of the excess padding in GTK apps comes from the Adwaita theme more than from the apps or GTK itself. After applying a GTK theme that cuts the padding down to more reasonable levels, I find that GTK apps are actually pretty nice and generally handle whitespace better than their Qt counterparts (which often go the opposite direction, packing controls too tightly or arranging them somewhat haphazardly).

Agree that Qt would be more popular with a C implementation. The myriad language bindings available for GTK have gone a long way in boosting its popularity.