That glider is not fat/overweight. When gliders are fat they are big everywhere not just the belly. What does she weigh?
If there was only one lump on that side view, the one by the back legs, I would be tempted to say joeys. But, the front lump is to high to be a joey bump and it's distinctively a separate bump.
If it were joeys, the bumps would be in this area. You can easily rule joeys out by inspecting the male to see if he is indeed neutered.
Because she is sleeping separately, and the front lumps position, I would lean towards something medical and suggest a vet visit. Sleeping separately is usually a good indication something is off even if you can't see any clear symptoms.