Buttons are too small, focus rectangle only seen at the rounded corners

Consider this screencast from Qt Creator 14's about dialog:

Notice that the focus rectangle is only seen at the corners of the buttons. 

Now have a look at how Windows Explorer's settings dialog looks like:

The buttons are less rounded, but still rounded, and the focus rectangle is inside the button!

This would explain why the Qt Creator's buttons seem so stretched.

Lastly here is a screencast from the Windows Notepad where the focus is around the buttons:

Qt Creator started with -style windowsvista:

This looks like the Windows Explorer above, and doesn't look like having small buttons at all.