A little correction, if I may.
I think we should consider the kynetic energy. That is Ek = ½mv2...sorry, I cannot write powers, so rephrasing it's actually (m * v * v) / 2. This is what I remember from many, many years back

.
This formula tells you that actually adding speed to an object will benefit you much more than increasing mass. (This is why you will see a small malinois taking down a big helper and sometimes a big mastiff won't, although it does pack significantly more power once it got a hold of you.) Mass appears once as a multiplier, velocity twice. However, and here's the catch: there's just so much speed you can add using your muscles. And with each swing you will find it harder to use the same velocity. But, OTOH, what you can do is to increase the mass. There's a point where increasing the speed provides diminishing returns, that is, because you are a human, and not a robot. You can imprint only "so" much speed to an object. That is why one will read again and again that with a kukri you let its own weight (gravity) do the work...and the flick of the wrist at the end. Because it is much, much more practical to lift a kukri and let its weight do the job...than using your muscle. Gravity wins each and every time here on Earth, in the long run, so why not use it?