What one has to realize is that *all* the "standard" widgetsets depend on external libraries: GTK, Qt, Windows, Mac, ... and they are all rather "heavy"; it can't be otherwise since they have to be generic enough to catter to all kind of applications.
The perceived "heaviness" of, say, Qt or GTK arises only if you need or want to install/use them in environments in which they are not the "native GUI". In those cases they have to include lots of functionality which in their native Unix are implemented in the OS or the low-level graphic layers. Hence a GTK or Qt application will always appear slower and will display more "quirks" in, say, Windows than a "native" application.
Whether that matters or not is a question best left to the planning stage of each project and depends on lots of considerations: portability, responsiviness needed, etc. There's no one-size that fits all.