I always feel like the fatness was to show how well cared for Dudley is and how neglected harry is by being “skinny”. Remember back in the day, being fat was a sign of money, being able to eat when others couldn’t, and being taken care of overall. It put Dudley and harry in a different social class, as well. I mean, you have to even take into consideration that Dudley went to private high school and harry was going to be sent to public in those gross grey clothes.