If the guy pounds him and he stumbles back to catch his balance or even goes to the ground like a normal person instead of like a soccer player - he might get the call. But when you exaggerate the amount of force in the contact and act like you just got hit by a charging Rhino - the refs can see that.

Actually - when Zac was a skinny toothpick he might've gotten away with those flops. But now that he's put as much weight/muscle on as he has - he can't.