Zimmerman chased the kid down... had the kid shot Zimmerman, stand your ground would have applied perfectly.
Where do you get the information that "Zimmerman chased the kid down"? I only see speculation from people who were not there.
From what I read, Zimmerman is about 225, and a bit pudgy. Trayvon Martin was 6 ft 3 (according to a poster) and about 140 lbs.
My guess is that Zimmerman would have a hard time "chasing the Martin down", at least with both of them on foot.