If there was such a thing everyone would use it and there would be no point in using any other.
"light" is a subjective term indeed. A qt based program can be light depending on how much qt do you use. Neither QT or Gtk+ are monoliths, they are composed by lots of libraries. You get what you use.
Other toolkits are tk, fox, qtopia, ncurses (text), openmotif, sdl, allegro..., there are many more. They are very different in their purpose and functionality, it just depends on what do you mean by "toolkit" and what do you expect from it.