Sledgehammers do about the same amount of damage as Maces and Irons for 3x the weight, I don't know what calculation you did but they are really really way off.
Sledgehamer = 1 Concussion, 1000hp, 30kg
Iron spike = 0.8 Piercing, 0.1 Concussion, 1000hp, 10kg
Mace = 0.5 Piercing, 0.5 Concussion, 1000hp, 10kg
Since concussion damage is somewhat weaker than piercing for some reason, one could argue that Sledges do even less damage than Maces and Irons too.