At 5'9" you have great physical advantages and you say you are rock-solid so you are probably strong enough and more; the goal of boxing training is not to look like Mr. Universe and you do not have to lift your opponent overhead, just punch him. You are 16 so it is quite likely that you'll naturally put on weight within the next few years and if not maybe you weren't meant to so why give up the advantages you have at bantamweight to become a lightweight?