If the idea is that it's about mass, everyone should just add lead weights to their short spear to get the damage up. This doesn't seem like it would be a good idea, though, in multiple ways. The force you can put into a stab depends on your physical strength, so F=MA just means that the heavier spear will move more slowly and hit with the same force as a lighter one (assuming it's at least heavy enough for you to put all your power into).
You want your weapon to be quite light - you hit with the same force, but more speed.