Why was Jem so judgemental about Dill saying Jem's full name? Remember to use quotes!

Jem is put off by a younger child giving him a hard time about his name. The boys are indulging in banter, nothing more, nothing less.

Jem scowled. “I’m big enough to fit mine,” he said. “Your name’s longer’n you are. Bet it’s a foot longer.”

perfect . . .