There is a fine tradition in animation, both American and Japanese, of prepubescent children being portrayed as about half the size of adults and teenagers. One of the older anime that was adapted into the Robotech franchise had 16-year-old characters that were basically adult height, but also 13-year-olds that were…

We don't actually know that. Our viewpoint character doesn't go to school and never asks follow-up questions. Everyone in Beach City treats the CGs as surprisingly mundane.