He asked "do you plan on getting fat?" Sensible question,  getting annoyed at what he said sets those kind of alarm bells off. She replies "no, but dumping something because they got fat is a middle school thing to do". She is absolutely going to be fat. To put ot another way, she identifies as a fat person. Eother the bf thinks she's not fat when she is, hence the reaction, or she believes weight gain is unavoidable.  Tldr: she gonna get fat