Girls are always girls either they are fat or they slim. They will ask for more even if you are doing best for them. You will have to understand the logic of girls even though girls do not believe in logical things but still you will have to understand the logics of girls.
If you call them fat they will be like yes I know I am working my physic but deep in their heart girls will be abusing you. Even if you call fat to a fat girl response will be same. Their response might be much curtail than this. Girls can come to you with the intention of killing.
So first thing you need to know do not comments a girl fat not to fat girl either. In case if you did it by mistake then your only chance to survive is apologize or straight forward say to them I was joking. She will be like I do not like these kinds of joke and do not do it again if you want to remain my friend.