First, have your vet come out & do a thorough exam. Any possibility she’s in foal? Stranger things have happened. That could explain the big belly. Then the current ration might not be meeting the needs of two. I’ve known people who changed boarding barns, only to discover the mare had a “romantic encounter” before leaving the previous barn. Or a mare was sold because they didn’t want the responsibility of the foal.
Feed to try – Nutrena Pro Fuel – high in fat, but low in starch for hard keepers. Also try adding Nutrena empower boost – a good and safe fat that always packs the pounds on my horses.
Is she finishing her hay? If she has trouble eating her hay, you can try soaked alfalfa cubes.