All KDE apps appear to set USSize. (Try xprop, it says "user specified size") I assume either qt or kdelibs is doing this. This is broken; USSize should only be set if the user specified the size, for example via the -geometry command line option. The USSize flag should not be set.
Notified qt-bugs to find out if there's a good reason for this.
it should be fixed in next upstream release