Qt Creator's UI looks unusably scaled

Good commit: 9d8441986e7e1ca74d1b45217e3a0b85463a165c
Bad commit: e68327666f36979f4c1fab8015df825e10eb0e0f

When starting Creator on my Linux machine, the UI looks too weirdly scaled to be usable. Please see the attached screenshots and compare the Qt Creator screenshots with the taskbar at the bottom and the Qt Designer screenshot for comparison.

And it's not that everything is big. If you compare to the creator2.png screenshot, note how the text in the editor is small.

No combination of environment variables seems to produce the correct UI.

$ xdpyinfo | grep dots
  resolution:    96x96 dots per inch
$ xrandr
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
eDP1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 293mm x 165mm
   1920x1080     60.00*+  48.00  
   1400x1050     59.98  
   1280x1024     60.02  
   1280x960      60.00  
   1024x768      60.00  
   800x600       60.32    56.25  
   640x480       59.94  

(so calculated DPI is 1920/293*25.4 = 166.44369)