with a heavier weapon, you can continue applying force throughout the swing, increasing its energy and the force behind the impact. With adamantine, its so light that the swing speed would be capped at how fast you can move your arms, giving it a maximum energy. In every possible case, this is still enough to cleave given its sharp edge. However, if for some reason it could not penetrate something due to its low mass, you could place the edge on the target's surface and apply direct force, simply using it as a wedge. At that point the low density isn't a factor at all.



In terms of cutting power it would go like this. Steel axe no swing (direct force) < Steel axe after a swing < Adamantine axe swung < Adamantine axe no swing (direct force).