I am not fat but I completely understand what you must be feeling right now and to be really honest you should tell them that it’s not funny calling you fat always. What I really feel you should do is talk to them and tell them how you feel. I am sure they will respect your feelings and will stop.

Hey! Being called out on something we are really insecure about actually hurts, i am sure you must not be feeling good about it. This is a common misconception that picking out on somebody means targetting their weakness, what you can do in such situation is that you can talk to your friends that this bothers you in the long run. They are your friends they will understand, tell them you don’t want it to effect your bond with them. You should not feel bad about the way you look, that is who you are. You have to first accept yourself and understand that if you are happy in your skin people will see that confidence and not point that thing out. And even if you also deel that you should also change the way you look, then you can work on yourself and appreciate yourself about how you are feeling now and track your progress. Set small goals for tourself and work on it.

It happens bro, it happens and happened with me too, being fat, isn't wrong, its being healthy, you should stop thinking like it is a minus point, because one day all these doesn't matter.