User blog comment:Miskos3/Fairy Tail Episode 266: Fairy Tail Zero: Fairies in Your Heart, Review/@comment-7418318-20160109084208/@comment-1776824-20160109084450

Hiro is just playing with the fact that the human head is nearly its full adult size (in regards to cranial capacity) at that age. That's why most kids look like they have big heads irl.