When creating the game in Unity; if you set the aspect ratio to 800x600 and put everything within that view it could help you.
I set the aspect ratio differently myself on purpose. I changed the canvas to scale with screen size, set the ratio to 600 x 600 and on the build settings set it to 400 x 400 to make it a smaller player. I think it's just the webgl's full screen that's causing the problems too because itch's full screen works fine. I can try that though, thanks :)