Even though kids eat like crap. EVERYTHING is fortified with what humans need to grow.  Take a pig, feed it junk , along with the correct vitamins/minerals it will be bigger than a corn-fed pig, given no minerals/vitimans.

Most mammals have an optimum window for growth, , given the correct amount, or over, they will grow bigger .

I have a nephew that is 6'8" , 325# freshman in HS. He is over weight but is also a huge frame. 
A S-i-L that is 6'10", his kids are young yet, but they are a bit bigger than my other grandsons of the same age.