Not entirely surprising, let's look at basic physics. If force is = mass x velocity we can see a relationship where both mass and velocity can matter.
Power isn't the same as force, but is an expression of force applied in a specific instant or extremely rapidly. So power = force x velocity.
Velocity is important in both equations so a slightly lighter man- still with significant mass- able to move explosively seems to end up with the more powerful punch on many occasions, since velocity seems to be the most important factor, assuming you have enough mass to be begin with*
* and lets assume ridiculous mass extremes are curtailed by the other facets needed at top level boxing like endurance, movement, co-ordination, skill and agility etc.
** The equations also ignore the fact that a more agile boxer may be able to move rapidly into more awkward positions for the opponent in able to actually land the power shot whereas a big slow giant may be just too predictable to ever deliver the power effectively.