You'll need the latest update of Windows 10 (1809) to reproduce this.

1. Open the start menu.
2. Type "text".
3. Click "Make text size bigger" from the search results.
   You'll see a slider for setting the text size on your system:
   
4. Move the slider to a higher value and click "Apply".
   Texts in the system will be resized, but Qt applications will not change:
   

The texts in Qt applications should be resized according to the setting.