Corpulent is a formal word that describes someone who is very overweight. You might describe Santa Claus as a corpulent man with a ruddy complexion and a jolly grin.
There are certainly many other ways to describe an overweight person: obese, portly, and stout are only a few. Corpulent is from Latin corpulentus, from corpus "body." The Latin suffix -ulentus, corresponding to English -ulent, has the meaning "full of, having in quantity."