Is genetics really the only explanation for a feather fisted fighter?

It has NOTHING to do with your muscle size. Explosive types of muscle can help. BUT EVERYTHING IS IN THE WAY THE BONE STRUCTURE IS SET UP, and how the muscle is set up on top of it. Your biomechanics. Look at a video of Donaire punching a bag with UFC's Mark Munoz (i believe that is his name, he is a famous pinoy mma fighter). Donaire is a little guy lanky no muscle on him, Munoz is 6'1 or so and 200+ pounds, and looks like a beast. Look at the indentation Donaire leaves on the bag, and compare to the nothingness left on it by Munoz. You can even hear the difference. You have to be able to generate power through leg/hip/upper body rotation. This is explained by biomechanics. People can train, or take steriods to increase their muscle mass, but their power will only increase by a tad. Its like throwing a 90+ mph ball, take 1000+ gym rats who juice and none will do it. Training and/or PEDS can increase power by single digit percentages. The reason why Steward helped Hearns with his power was because he didn't know how to use his leverage. But he always had it. Steward was great for punching technique but he couldn't make a puncher out of a guy like Mayweather or Malignaggi. Its just not in the biomechanics (bone structure which includes density, but its more of the arrangement and angle of the bones in addition to how the muscles on top of it function with these angles and arrangements).

It comes down to style,Paulie for example doesnt sit on his punches as he is on the move and add in his fragile hands he is looking to score and move. Bradley isn't feather fisted at all but he isn't a brutal puncher either

The few trainers who have coached me over the years have always been surprised that while I can punch holes in a punching bag and break people down when sparring, my power is less impressive when on the mits. Namely because I'm scared of making a mistake and tighten up because I'm thinking too much. However, when I'm sparring, I feel much looser, and in control. I relax and thus my technique comes to the fore allowing me to punch harder. There are literally a million reasons why some fighters can't punch hard. I don't think it comes down to build or design. It's a combination of factors. Mallignaggi's brittle hands have subconsciously made him into a feather fisted fighter. He rarely lets go of his right hand, and because he can't fight opponents off him, he has to move and jab unable to set his feet which further reduces his ability to knock opponents out. Sure, he isn't a born puncher, but if it wasn't for his hands I'm sure he would have stopped far more opposition. Matthysse knocks people out because he has excellent technique, a strong bull like build, and always sets his feet. Maidana is similar.

We all have fast twitch and slow twitch muscles. Fast twitch muscles aren't the "sexy" muscles but rather the effective muscles that create short quick explosive bursts. It's always been said that in boxing you don't need to be über diesel like Berto or Gamboa. What dictates your speed is a specific muscle type that really makes your physical look irrelevant. But there are other factors such as nutrition, supplements, metabolism, hydration, conditioning, etc. and when it comes to effective power you have to account for balance, form, and technique. That said 'pure' power derives from having powerful fast twitch muscles. So yes; it is genetic to a large extent.

Get in the ring and say that..

Boxers are trained to punch, yet some end up like Bradley (who is muscled as hell) or even as bad as Paulie's punching power. Do they not have bad intentions, or is their genetics holding them back? In the case of Bradley atrocious mechanics and failing to sit down on his punches have detracted from his striking power. Even he acknowledges it. Beyond that I go back to my original point. Bradley, the mass of muscle that he is has always been more concerned with quantity than quality. I'd say that's another way of decoding this question.

Difficult to say. But if there's any controllable influence I've often theorized it depends how you train and develop your body. If you put as a bottom layer, a foundation layer; speed, quickness and emphasis on scoring you will develop as that type of puncher. If you put as your foundation layer an emphasis on loading up and hard hitting you will be that type of puncher. This has been my personal experience in-the-gym.

Boxers are trained to punch, yet some end up like Bradley (who is muscled as hell) or even as bad as Paulie's punching power. Do they not have bad intentions, or is their genetics holding them back? Paulie has had hand injuries throughout his whole career, which is why he lacks punching power. Same for Mayweather. His last "real" KO was Ricky Hatton, years ago IMO, punching power has more to do with technique than genetics. If it were genetics (physical attributes), then only muscular guys would be power punchers (or guys with heavy bone density). Matthysse isn't ripped but he has amazing punching technique, so he gets the absolute most out of his shots. Bradley smothers a lot of his punches, and also doesn't use his legs when he throws.

A friend of mine is built like a twig, yet he hits like a train.

Genetics plays a part, but I think it comes down to being open-minded and able to constantly try out new techniques and forget what doesn't work for each individual fighter. Once you experience and become familiar with the feeling of letting go a solid power punch, you can identify ways to improve your technique so that YOU can punch harder, like the most fluid way to shift your weight back and forth, what time is best for you to release your punch and tense, etc.. Some people just try to copy their trainer's technique, or a boxer they want to punch like. If they're built differently, they won't be as effective with that particular punching technique. Some people stay punching softly because they either don't think - or understand how - they can improve, or they just don't feel like they need to punch harder.

The Zodiac determines your punching power.

Training is a factor..Hearns did not have that kind of punching power till he hooked up with Steward.

Bradley to me is more focused on defending, the only punch he really puts serious mustard on is his overhand right, you don't see Bradley shift his weight into the left hook the way Cotto did at 140-147, or even Ricky Hatton. Bradley isn't feather fisted though, I used to get into a lot of debates when it came to that. People forget he stunned Alexander multiple times, hurt Kendal Holt to the body, hurt Peterson, dropped/ hurt Witter, had Abregu hurt, hurt and also dropped Edner Cherry if I remember correctly...list goes on and on.

Boxers are trained to punch, yet some end up like Bradley (who is muscled as hell) or even as bad as Paulie's punching power. Do they not have bad intentions, or is their genetics holding them back? Its the biomechanics of their muscle and skeletal structure. They just can't generate the amount of leverage needed to generate power, and also don't have the bone density to be heavy handed. That is why all of this ped talk is hilarious. I believe most fighters are on something, but the amount of enhancement at least in the power department (stamina can go way up with EPO for example, and punch resistance can be increased through more musculature) is minimal. I know of people who are very muscular and have taken performance enhancers (steroids, growth hormone) who can bench press a lot of weight but can't pop a grape with their punches. If you do not have the biomechanics to be a big puncher all of the drugs in the world can't change that.