Someone brawny is bulky and muscular. Most football players are brawny.
Brawn refers to muscles and physical strength: people often contrast brains and brawn. Therefore, if someone is brawny, they have large muscles and are quite strong. Many athletes are brawny, especially ones such as weightlifters and football players. Tennis players and jockeys are usually smaller and not so brawny. A brawny person is big, strong, rugged, and tough. If you'd like to be brawnier, go to the gym.