Trayvon Martin was actually 6'3" and a football player.

Apparently the 6'0" measurement is from an incorrect measure during his autopsy.
and
http://en.wikipedia.org/wiki/Shooting_of_Trayvon_Martin#Trayvon_Martin


So he had a six-inch size advantage over Zimmerman.

Another 911 witness to the confrontation said that based on Martin's clothing, he identified Martin as the one who was on top.