I can’t see Zimmerman fearing for his life. While some high school kids are very strong and big, he was not as big as Zimmerman at all.
You are being deceitful with the facts. Martin was 6’1″, while Zimmerman was 5’9″. However, Martin did weigh more. So, you can truthfully say that zimmerman was “bigger”, but in the ways that count, we should say that martin was taller and in better shape. zimmerman is just short and fat–like most people with the name zimmeman. (I’m not nearly 5’9″-that must be because he’s a goy).
Moreover, you have no idea what happened. If martin was on top of him punching his lights out (I’m making this up), then of course he could have been afraid, even if he was bigger.