If a zombie/zombie pigman picks up a sword, or has one, the damage they deal is their normal melee damage + the sword damage, so if their melee damage is 2, and the sword damage is 2.5, the damage they deal will be 4.5, instead of 2.5 that it should be.
Steps to reproduce:
1. Create a survival world
2. Find a zombie with a sword/give a sword to a zombie
3. The damage will be the difficulty damage + the sword damage
Linked issues
is duplicated by 2
Comments 3
If it weren't like this, zombie pigmen and wither skeletons would be extremely weak. Rather, they hit high because they have a high base damage. Remember, they aren't human (anymore?), so they may simply be more powerful than you, and thus able to deal more severe blows with their weapons. If a human with stronger muscles wielded a sword, logically, they would be able to swing it harder than a weaker human - it shouldn't be any different for monsters which are already inherently stronger than the player.
Incidentally, I don't think the bug with skeletons doing fist damage with swords has anything to do with this behavior. If it did, skeletons would do their fist damage plus sword damage. Skeletons seem to be an anomaly; zombies, wither skeletons, and zombie pigmen all deal more damage with more powerful weapons.
N.B. if the game worked the way you proposed, and you gave a zombie a wooden sword on normal, they would not get stronger. If you gave it to them on hard, they would get weaker. Really, I think the way it is now is intended.
I think that's the point man. In my opinion mobs need to more damaging otherwise there is no threat.