Boxer's tend to generate more power with less wind up, their punches come at different angles and are set up with a different base than the majority of MMA strikers. The energy for a punch really starts in the legs and because there is no kicking or take downs in Boxing, the boxers are most often in position to generate more powerful punches. Also boxers tend to keep their power punches short and crisp with the power coming from the torque from their whole body moving, on average MMA punchers tend to have much longer unrefined striking with less power and torque. Obviously there are a lot of powerful punchers in MMA but on average the boxers generate more power especially with those bigger gloves, more surface area and hand protection.