Not really a real-world app, but a simple QtQuick with Qt3d app worked pretty well. When it comes to app sizes, a vanilla Qt app produces an apk file of ~15MB, which is reasonable.

I had troubles accessing external storage (the SD-card), and ended up writing quite some ifdefs to fix that. Probably the kdab folks have proper solutions to most common issues, but at the end of the day the idioms between desktop apps and mobile apps are quite different, and you will end up wasting time to fix those issues if you really believe in having a single code base for cross-platform apps.