In "...\Game\MapDefs.xml" there's a pair of lines for each map size <MinCameraZoom> and MaxCameraZoom>. It's an inelegand fix for you, but I'm not seeing a way to scale the camera zoom through the size of the UI.
While this would seem to be an oversight, it actually makes sense that they wouldn't (or couldn't) do this. The map and environment is modeled with all its constraints - including the bounds of the camera. The UI is just an overlay that goes on the camera. Changing the MaxCameraZoom would require reloading the entire map if that variable isn't baked in to the map on game start (which I think it is).
Hope this helps, even if it's not the answer you were hoping for.