Note that the weight of the axe head is not particularly important (for power) in a pneumatic axe, as long as you have good ram speed. A lighter axe head also tends to make the robot jump less.

However, on an electric axe, the only way of getting a good level of enery into the axe is by having a very heavy head on the end of a long shaft. The motor is producing energy at a constant rate, so the longer the swing time, the more energy will be in the axe head at the end.