My point is that a lean build of GTK3 doesn't require more than GTK2 right now does. In the port linked to, libepoxy and libcolord can be avoided. And when you do that, it's down to what GTK2 depends on in its default build config on most Linux distros. If you could disable ATK, you could shave off some more, but GTK3 doesn't build without ATK.
Qt5's base memory overhead is big (minimum 25MB for the simples hello world window you can create). The modularization of Qt5 is a good idea, but it also makes use on Linux distros a pain, because it's too many modules, like the mess some Linux distros create when they split core Perl into a gazillion packages.
https://www.freshports.org/x11-toolkits/gtk30 It's "lean" by default, you can't even make it "not lean".
They're big, but I wouldn't call them "user-unfriendly and dev-unfriendly". QML/QtQuick is the best GUI development thing ever.
P.S. I was building Firefox from ports for a while, just to get GTK3 support earlier than they turned it on by default :-P