Precocious puberty is a thing, and even some girls who go through puberty at the normal age can experience such rapid and extreme breast growth that they would qualify for large cup sizes that are rare even among adult women (like, adult head size!) before they even hit age 13 (which typically causes a lot of medical problems due to their bones and muscles not matching pace, thus leading to chronic pains and permanent deformations). Japanese Fictionland is just applying the same kind of breaks from reality that it applies to a lot of other things (e.g. why people often get punched a hundred meters into the air with little to no lasting harm, despite how vastly unrealistic that is).
angrily tapping phone screen in the hope that the next non-existent pages will reveal that everything is a big ol' misunderstanding and cute rescue girl is actually the running kid's big sis or something
inhales ELEMENTARY SCHOOL??? What grade does elementary go up to in Japan because I am confusion.
Up to sixth grade, i.e. age 12-13 (depending on the kid's birthday, because the system is bone-headedly strict and doesn't care if you're only a couple months or even weeks away from hitting the legal age for starting elementary, which thus determines how old you are when you graduate from sixth grade).