background not scaling

[knotbin]

USING ELECTRON FORGE MAKER DMG

The background always stays the same size, no matter its scale, and there seem to be no options to change background size.
ORIGINAL BACKGROUND IMAGE:

When using the reccomended background size, 658x498, the background doesn't fill the full window, as shown here.

When the background image is scaled up, the window scales correctly but the image stays the exact same size.

Any advice on how to set the scale of the background image or at least get it to fit the window?