App windows go small after tapping app switcher button

[gpaitai]

Hi, I recently received a new head unit, JY-UQ135N4G (7in 2Din with physical volume knob). When I tap the app switcher button (the centre green button with 6 dots inside on the home screen) it resizes all the apps windows really small, centred within the screen. How do I get these app windows to fill the screen like normal?  I'm new to Android so may have inadvertently tapped something, so really hope this is a simple fix.