Corpulent is a formal word that describes a fat person. If you are trying to be respectful when describing a fat man, you might refer to him as a "corpulent gentleman."
There are certainly many other ways to describe a fat person: obese, overweight, 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."