That would explain it. I tried zooming both in and out, but no change. I find it odd because usually Unity games scale really well when fullscreened. Maybe you can figure out how to make the screen size more flexible post jam. Still frustrated I won't be able to rate it at this point, but fortunately it sounds like enough other participants have big screens :)
P.S. I had a slightly similar frustrating fullscreen issue with my software GDevelop at one point that when I tried to fullscreen my games, it would only stretch the window to the right instead of increasing the size of its contents and there was no intuitive way to fix it, so until I finally got some tips for another dev, I just couldn't add that feature at all :(