You should not comment on other people's bodies.
Gerard is about to play tennis and sees a very overweight person playing on the court he reserved.
Gerard wants to get the person to leave the court, so he can use it.
Gerard kindly explains that he has the court reserved and asks the person if they can wrap up playing soon.
Gerard calls the person a fatty and tells them to stop wasting time on a court that a fit person could be using.
The person understands, packs up their stuff, and moves to a different court.
The overweight person is furious and refuses to leave the court.