Well, it could be due to a combination of factors. You might have had an irregular eating schedule, lots of sugary drinks, and spent too much time sitting instead of being active. Stress and lack of sleep can also play a role in weight gain during high school.
Some of the common ways girls get fat in stories are through unhealthy eating habits like snacking constantly on junk food. Also, they might have a sedentary lifestyle, spending a lot of time sitting and not being physically active. Sometimes, certain medications or medical conditions can also cause weight gain for the girls in the stories.