Why is Jem trying to gain weight? And how? in ch.26
The reason that Jem wants to gain weight is that he wants to go out for the football team. He tried out as a 7th grader (this year) but he was too skinny and the coach wouldn't let him do anything other than carrying water buckets. So he feels he'd better gain weight.
Specifically, the coach tells him he needs to gain 25 pounds by next year. So he is trying to overeat. When Scout sees him, he has eaten at least a dozen bananas and has drunk a bottle of milk.