Lol Vicious... show me a 2h that weighs 10kg (except Mauls). A normal 2h weighs around 2kg, maybe 2.5. Wielded with 2 arms, there is way more power behind as there would be with 1h. Kinetic Energy comes from speed, you can wield a lightweight 2h faster than a lightweight 1h, which means there's more dmg from the 2h. So no, 1h should not be able to do as much dmg as a 2h weapon in general. The problem is, if both player slash at the same time, the 2h just has the advantage of range, that's why his hits almost always connect earlier.
As you said though, most 2h seem to aim for the feet instead of the head, opposite to 1h, who almost always have to aim for the head. That makes up for the difference in damage dealing.