I am a big fan of Dungeon World. One of its greatest strenght is the way it connects the fiction to the mechanics : in DW, the mechanical rules will fight you if you think about the game in an abstract instead of narrative way, not the other way around.
But the way monsters’ HP and damage are calculated goes directly against this. The official method makes base HP and damage a function of the “social organization”, so a lone monster gets more HP and a bigger damage die than a monster usually found in large group. I understand how that makes sense in a “gamey” way, but that does not reflect what the PCs are seeing. If I see a Salamander (a 10 feet high creature with the torso of a man and a big snake tail instead of legs wielding a flaming spear), I expect a mightier opponent than a Halfling Thief. Yet, the thief has 12 HP while the salamander gets 7, because salamanders are usually in large group while the thief is usually working alone. A Dragon Whelp (a dog-sized dragon) gets 16 HP, because it’s also usually alone (but less puny than a halfling.) The problem is similar with damage. That’s unfortunate, because everything else works perfectly fine to model the fiction : more HP if it’s really tough or big, less damage if it’s tiny or hates violence, etc.
To make the monster stats model the fiction better, I think that basing the monster’s stats on its size and would be the quickest and easiest way to go. To keep the conversion simple, I would do it like this :
- Tiny : 3 HP, d6-2 damage
- Small and Medium : 3 HP, d6 damage
- Large : 6 HP, d8 damage+1
- Huge : 12 HP, d10 damage+3 (I already talked about this, but I also would also play those monsters in a way where dealing any kind of damage to them require something special, like getting to a weak spot or using siege weapon; think “Shadow of the Colossus”. Every fight with a Huge monster is a puzzle first. That’s already encouraged in different places through the rulebook, but I would make it explicit in the tag description. So I leave HP here for the sake of completeness, but I would probably not use it in game.)
All the other modifiers would stay the same. That gives us a quick and easy way to modify monsters in the core rulebook (and any monster made with the official rules) :
- Tiny, Small and Medium monsters : reduce the damage dice 1 step for Group and 2 steps for Solitary (no lower than d4); decrease HP by 3 for Group and by 9 for Solitary (minimum 2 HP).
- Large monsters : increase the damage dice 1 step for Horde, reduce it 1 step for Solitary; increase HP by 3 for Horde and decrease it by 6 for Solitary (minimum 5 HP).
- Huge monsters : increase the damage dice 2 steps for Horde and 1 step for Group; increase HP by 9 for Horde and by 6 for Group (minimum 10). There are no Huge Horde monster in the core rulebook, by the way.
The result is that all non-Huge Solitary monsters are going to be weaker and that fighting a Large Horde is going to be basically suicide. I think it’s totally reasonable.