The font sizes occasionally end up way too large or way too small considering the scale factor on Windows. This seems to occur especially after changes to scale factor and/or if different displays have different scale factors.
For example if the main display scale factor is increased above recommended the font sizes seem to decrease on all monitors. This affects even Qt applications started after the change.
Alternatively closing internal display of a laptop might throw the font sizes off in Qt applications on an external display.
This kind of behavior can be reproduced with Qt Creator as well as Qt examples such as "examples/mainwindows/application".