oh, I think you are using gamemaker, I might be wrong, if you find a way to fix the fullscreen issue let me know.
Viewing post in Ecora's Echo Chamber jam comments
This game that I made https://angelkof15.itch.io/space-colors-beta doesn't scale properly when you go full screen on itch.io, only fits like 40% of the screen the rest are black bars, and I really would like to fix that. I notice your game has a similar issue, there are small black bars when you go full screen.