What does this mean?

This is a joke

If it is posted by a girl, it means that she has small breasts but she doesn’t want to admit it, so she says that she is still a child because she has not started to develop, so her breasts are small. But this is just a joke. A self-deprecating joke shows that this girl is more open-minded and humorous, but don’t make fun of others

If this is posted by a boy... then I really don’t understand it!