science can explain this, acceleration * mass = force.

Quicker you can move your arm and more of your body weight behind it the harder the punch. SIMPLE... sort of.

Im an ex sprinter but I did play and train with some boxers many years ago, went to a science place and had some punches measured for speed and impact force. The boxers outscored me, by a good margin. They hit hard, really hard. Now I have a couple of black belts in kung fu and karate. The technique was similar but also very different.

My kung fu instructor talked about a punch should be like "an iron chain with an iron ball on the end" The karate guy told me it should feel like an iron stick. The boxers when I talked about this with them said, turn your hip and punch through their face, and if you use your legs you should punch with the mass of the world behind you. (not true but not a bad training lesson)

There is also for want of a better description magic at play, some boxers just have heavy hands. One of the boxers I was training/playing with punched me on the shoulder. Just a jab and my jaw rattled.

Think about a domestic cat, crazy little wannabe tiger, watch their paw speed. Crazy quick. will hit a toy 4 or 5 times a second ...bap, bap, bap, bap. But so little body weight behind this. Now watch a tourist on holiday that stands next to an elephant. One swing of the trunk and tourist is knocked over, elephant ways more than a cat.