Going full screen doesnt fit the picture to screen
 

Hi,
I might've accidently pushed random buttons or something, or this could just have started happening for other reasons. Everything used to work fine.

Now when I go to full screen (any method) the picture size is reduced to about 1/4th of my screen, all borders black. Window size doesn't affect the full screen. Movie mode and auto-pan (O) fix it for a moment but when I leave full screen and go back, picture is back to 1/4th.