Generally I think this has something to do with average age of model trained vs age of depicted person. Humans have large heads proportional to their bodies in youth and them averages out. On top of this another commenter mentioned art. So many anime/cartoon characters have larger heads to show the expressions and in turn show a youthful nature in these characters.

Similar to “the average number of arms is less than 2 because there’s greater chances there will be people with zero or one arm, rather than an extra”. Your model will draw on its wealth of samples where the head is disproportionately larger, instead of proportionate - because it was probably fed more photos of hat age range and doesn’t know any different.