There is no easy answer for this and I know thats not what people want to hear but its the truth; that and everyone is different. I grew up boxing and playing American football. In both I was considered well undersized -- specially for my age -- until my senior year of high school. Oh, and comparatively I too have always had smaller biceps than many of my friends. Regardless of what I could or couldnt do in either sport or how good/bad I was at either, the one thing I COULD do in both sports in far excess of my weight was BANG. And I was never a case where I weighed less than others because I was short for my age, I was well above average height for my agae and didnt start filling out my frame until senior year high school. I could crack in either sport without anyone ever showing me how or knowing technique, I could just do it. Some of the basic elements in the ability to punch hard are either fast hands/quickly accelerating hands, fluid hips for transfer of power and everyother chain reaction between joints and muscles that lead to as fluid a transfer as possible, a strong base with excellent balance, punching through intended target as opposed to punching to intended target, leverage, timing, accuracy, commitment, and an unwavering self belief/confidence in your're power. There are two types of punchers: explosive punchers and heavy handed punchers and fast/slow twitch muscle fibers and an individual's personal collection of the two dictates what type of puncher, but the first things I listed -- at least some of them, are usually universal. I was a scholarship football player my freshman year of college and we had some incredible natural athletes. I also did a post-grad year (prep school after my senior year of HS) because I couldnt pass the clearinghouse academic standards needed to accept either a scholarship(div.1-1AA) or a grant(div. 2.) The conference I competed in (NEPSAC) is the most competitive in the country in basketball and one of the most in football, and every player on the postgrad football and basketball teams were athletes going on to play div. 1 sports, and usually high level. That said, at both prep school and college there were plenty of guys who had at least novice level amateur boxing experience and like I said there were some pretty remarkable athletes at both schools. I'm not saying I was a better boxer or better at whatever sport than most of these guys, but p4p, I hit harder than all of them -- in boxing and football(if they played it.) So, there are some basic elements that should help a boxer punch harder but at the end of the day everyone is different and so everyone has their own recipe and some can just do it while others -- no matter how gifted -- just can't.