OpenGL? "Lightweight"? Sure, I suppose because it's all implemented in the system, you don't have to redistribute much, but have you actually ever written anything remotely complicated in raw OpenGL? For anything resembling a GUI, the poster is going to spend months of writing low-level code that's been done a thousand times already.
Qt is heavy, but it's heavy for a reason - it includes a very nice set of tried-and-true widgets, with all the nice features and weird corner cases thought of already. It's also fairly speedy, and even more so if you use QGraphicsView, which can be optionally accelerated via OpenGL for even more speed.
Qt also has nice support for custom widgets. You can subclass any widget, or QWidget, and make anything you want. You can even integrate your custom widgets with Qt Designer, either by promoting a placeholder widget, or writing a Designer plugin so your widget is WYSIWYG.
OpenGL is so low level that everything I talked in the last two paragraphs is completely beyond its scope. Even font rendering is rather arduous, and good luck with nicely word-wrapped, formatted text.