How to throw a more powerful punch?

So the other night, my college suitemates and I went to the local pub to unwind a bit from the week’s work. While we were there, one of them got up and put a dollar into the punching bag machine… enough for each of us to get one good hit in.

He reeled back and nailed the punching bag, sending it flying up into the frame. The score counter read 898 our of a max 999. Then, my roommate went and scored 870. Another of my suitemates followed with an 880. Finally, it was my turn. I focused, balled up my fist tightly… reeled back, and swung… 680.

An old man then came by who was much smaller and scrappier than I. He popped a quarter in the machine, wound up, and slammed the punching bag, scoring an 860.
Not cool.

For one, I’m much smaller than most of my suitemates, and I don’t have waifishly thin baby arms, by any stretch, but I’ve always been aware that I can’t hit very hard, and I’ve never been able to figure out why. My suitemates and I will often have impromptu boxing matches, and I almost invariably get my ass handed to me, even against the ones who are much closer to my own size.

It is my firm belief that every man should be able to throw a proper Ernest Hemingway-ian punch in addition to being able to wrestle bears and chop down trees with their face and I would like to learn how to do so.

I’m not weak, by any stretch, so I think it may be a matter of technique. In that case, what kinds of martial arts focus on hand strikes, particularly accurate, powerful ones? What are the best exercises to build bicep and tricep definition? Are any punching-forms more effective than others?

And don’t worry, all you handwringers out there… I’m not planning a punching rampage. This is for educational and recreational purposes.