I'm pretty sure this can be done, and it's like this:
By default these buttons are sized 28x15.
Nowadays you can additionally add 30x28 sized images.
You can add them to your skin using these elements:
FormMinimiseButton2
FormMaximiseButton2
FormRestoreButton2
FormCloseButton2
FormMinimiseHighlightButton2
FormMaximiseHighlightButton2
FormRestoreHighlightButton2
FormCloseHighlightButton2
Note that the original ones should be kept, and these new ones should be placed after the original ones in the xml.
Perhaps things turnout differently when using Windows scaling? I don't know.