If someone is heavy-set, their body is large and solid. They look strong, or they look fat, or a bit of both — the word is about the shape you see, not a number on a scale. A heavy-set person often has a broad chest, thick arms, and takes up space in a doorway. English speakers like this word when they do not want to say fat out loud; though actually it can still mean “quite fat,” just in a softer, more polite coat. You use it of people, not of bags or furniture. It sounds a little old-fashioned and is more at home in stories and careful speech than in teenage chat. Grammar is simple: a heavy-set man; she is heavy-set. You would not use it of a thin, wiry athlete, however strong they are.