“Fatso”, you ask? Why is Harry fat?

The Dursleys routinely forced Harry to go without food as a punishment. As a result, Harry's body adapted by storing extra fat whenever possible, to save up for times when there was no food.

Then, at Hogwarts, Harry was allowed to eat all the food ze could possibly want. This is a good thing: People need to eat a lot of food to recover from malnutrition. Typically, they gain a lot of weight at first, then eventually settle back to something like their natural weight, whatever that is for the individual.

Judging by the books, Harry is naturally skinny, so it's a little unusual that my Harry is still fat years later, in the present. However, periods of starvation can also have a permanent effect on people's metabolism, and chronic stress can cause significant weight gain as well.

Crabbe is obviously just being mean. But other people who might harass Harry for being fat might think that they are encouraging Harry to eat healthier. Those people are both cruel and incorrect: Harry isn't unhealthy because ze's fat – ze is fat because ze's unhealthy. Eating less would make zem even more unhealthy.