“...Adult cats typically weigh between seven and 12 pounds...” Not some breeds! Maine Coons are one of the largest breeds of domestic cat. Males weigh from 15 to 25 lb (6.8 to 11 kg) with females weighing from 10 to 15 lb (4.5 to 6.8 kg).[17]
May be he has a Maine Coon gene in there somewhere? Then again, did someone just stuff him?
He could have untreated thyroid issues, but my money is on too much nom noms.