I don't have any personal experience with Web builds. A bit of online searching turned up this:
https://docs.unity3d.com/Manual/class-PlayerSettingsWebGL.html#Resolution
It looks like you can set the resolution for the web build using the Unity Player Settings. If you specifically give it a resolution that you like, with the right aspect ratio, and make a new build, you might get the edges back. I don't know that you could re-submit for the Jam, but you could certainly update your project build, so that future folks could appreciate the whole thing :)