I'm going to tell my girlfriend she's fat
Why are you going to tell her she is fat?
Because she weighs around 50kg.
It is okay to be concerned about your partner gaining weight because of health concerns, but you should think twice about just telling her she is getting fat. She will be hurt and it may lead to a fight.
It's fine. I want to hurt her to prove a point.
Hurting her will not prove your point, it will make her withdraw. There are better ways to tell her you care about her weight and want her to be healthier, like asking her to go to the gym with you.