I hinted at it, but I'll say it outright: if the bodies of the kids are bigger than average, then it makes sense that their heads (and feet, for that matter...) will be, too.
Now, a small-bodied child with a bigger-than-average head is a different story. I think proportion matters. A tall, heavy kid with a big head is just a big kid. A short, lightweight child with a small head is just a small kid. <shrug>
On the bright side if you're into TV and movies, people with proportionally large heads for their bodies film well. Most TV and movie stars are walking lollipops: twiggy bodies and big ol' giganormous heads.
