Zimmerman is 5 feet 8 inches tall, 195 pounds at that time, Martin was 6 feet tall, 150 pounds. Mesomorph versus ectomorph.
I think Zimmerman either tapped or grabbed at Martin while he was still on the phone with Jeantel. She heard Martin say, Get off! Get off!, right as the mobile headset hit the grass. I think Martin layed out Zimmerman with a good, hard punch to the face that toppled Zimmerman backwards and down onto the sidewalk, where the back of his head snapped back from momentum and hit the cement.
Martin had damage to only his right hand, small abrasions to 2 knuckles, and no other wounds aside from the bullet wound in his chest. Other than when throwing one good punch right at or near the start of the fight, I think Martin became definitely aware of the pistol, either because he saw it, felt it against his body, or saw Zimmerman's hand moving to the holster as he lay on his back from that nose-breaking punch, which would scare the shit out of anyone, whether it's dim and dark, or lit by a street light. That motion is tell-tale.
Zimmerman says he struggled to find his mobile phone in the pocket of his pants. He probably did, but later on. What he was reaching for while Martin looked on was his pistol. Zimmerman knew that too, so the focus for both men was to get control of that weapon, with a lot of grabbing and wrestling and some lighter jabs or hard slaps to prevent the other from getting a grip on it. So think about that scenario - both men are desperate and frantic to get at that pistol. Either or both of them could be the voice or voices screaming for help heard in the 911 calls.
If only he was not in the habit to go grocery shopping with a holstered pistol, the only danger Zimmerman would have posed to anyone that night would've been maybe boring them to death. I seriously doubt he would have followed Martin so closely.
Seriously, this dope told the interviewer at the PDHQ that there was "no way, no sir" that Martin could have seen his pistol, because, "I walk around with it on me at Walmart all the time. Nobody has ever seen it." Um, actually, with that same jacket, all he had to do was be observed while reaching for a jumbo bag of chips at shoulder level for the pistol to show. I guess nobody ever found him interesting enough to glance at, or no one wanted to have a friendly chat with an armed shopper.