Armor Theory
The general idea behind a bladed weapon is to apply a lot of force to a very small area. The sharper (read smaller) the area applying the force, the more effective the weapon can be. Think of it this way; a round pipe can leave a red mark, a dull knife can leave a bruise and maybe break the skin, and a sharp knife can penetrate the skin damaging muscle and bone. The mass (weight) of the weapon also plays a part. The heavier the weapon, the more damage it can do. Think of the difference between dropping a 4 pound sword on your foot versus dropping a sewing straight pin! More mass means more energy, and that means more damage.
Now, the way armor works is supposed to be just the opposite: The more you can spread the force of the blow over a wider area, the less damage is done. Damage is also reduced if the armor can reduce the energy being applied to a small area. This is why a heavy helm or helmet is considered safer than a lighter helmet in most cases. The energy needed to move the great big heavy thing surrounding your head is subtracted from the energy actually being applied to your head, therefore your head actually remains on your shoulders!
Think of it this way. You have a croquet mallet, and you lightly swing it at a croquet ball; the ball move a little bit. Now, take the same mallet, swing it the same amount at a 16 pound bowling ball; the ball doesnt move, or moves so little as to hardly be noticable. The difference is that the heavier "target", the bowling ball, absorbed a bunch of the energy, so it didn't move.

Isn't physics wonderful?!!!