Power units are J/s, or kgm^2/s^3. Speed x distance is m^2/s. Speed of jab does not determine power, though it plays a role. Fighters often throw improper jabs in an attempt to be fast and end up snapping from the elbow, making them weak. Power isn't that important anyways, just means I can move you with the punch- doesn't mean I'll actually rattle you. Impact is what determines effectiveness, and that's the rate of change in momentum from connecting with your opponent: (momentum before contact - momentum after contact) / time. That means the less time it takes to lose all momentum in the jab (hence speed), the more the damage. And the momentum is mass x velocity, making speed doubly important. Mass plays a role but not as much as speed- when you throw from the elbow, you lose momentum as you carry along the extra mass between your elbow and shoulder, but when you throw from the shoulder you pack more mass initially with almost the same speed and only gain speed from there. That's why a lighter person may punch harder... buuut there are other factors that come into play, like mass being due to bone density (as opposed to muscle and fat) or muscle(as opposed to fat), as well as mass distribution. So there's a whole bunch of possible reasons said welterweight punches harder than said light heavyweight