It's probably because you're weakening yourself by burning more than you consume. obviously not going to be optimal. And 'power' in a boxing sense is a lot more than some math equation.
It depends on how many fast twitch fibers you possess. If you want to lose weight while trying to while trying to minimize the loss of power or muslce you have to design your workouts to be of short duration but more intense like hill sprints, Tabata intervals, etc. Limit the amount of roadwork(aerobic) you do as well.
dont get me wrong my speed has doubled since i started but i dont really knock the bag around like i used to. I lost 40 pound btw