I’ve been searching but not sure I understand what’s going on. When playing through my projector on 1x and 2x the screen is filled too and bottom it’s a standard 4:3 aspect so I’m happy with the bars left and righ. As soon as I play a lot of the consoles on 3x 4x and 5x the top and bottom of the screen isn’t filled. Can it be adjusted or is the the resolution of what my projector is receiving that cannot be adjusted?
Images are too small to really see what’s going on but it may be as you suspect, just how the projector is scaling things. Unless there are settings on the projector there may not be a way around it. On the OSSC, you can try fiddling with V.Active in the advanced timings.