On the whole, it more or less breaks down like this:
- large piercing/slashing weapons do more damage vs. large targets than they do vs. small/medium targets
- small piercing/slashing weapons do equal or less damage vs. large targets than they do vs. small/medium targets
- blunt force weapons of any size do equal or less damage vs. large targets than they do vs. small/medium targets
This general scheme makes sense to me. If you run a 4' sword through a human's 12" deep abdomen, the sword only rips through 12" of flesh. If that same sword is run through a giant's 24" deep abdomen, that sword rips through 24" of flesh, thereby doing more damage.
Conversely, a small piercing weapon, like an 8" dagger, in either case would only rip through 8" of flesh. For the human, this is 67% of the total depth of the abdomen, while for the giant this is only 33% of the abdomen's depth -- thereby doing proportionally less damage to the large target.
Similarly, a blunt force weapon should do, at most, the same damage to a small/medium or large target, and possibly less to a large target, since the force of the blow is the same in all cases, while the large target would theoretically have a thicker muscle wall protecting its organs and bones against anything that gets around the armor.
Here though, is the conundrum. All the old rules that I'm aware of (including Greyhawk, AD&D and clones like my favorite, Iron Falcon) that use this differentiation of damage between small/medium or large targets list sling projectiles (in AD&D specifically sling bullets) as doing more damage vs. large targets than they do vs. small/medium targets.
I can't get my head around this, since the sling projectile is a blunt force weapon, and a very small one at that. In theory it shouldn't really penetrate per se, and the normal principles of blunt force weapons should apply (i.e. it should do no more than equal damage vs. a large target, possibly less).
Even if I'm mistaken here and it does penetrate, perhaps due to its high speed, I don't see how a small, blunt projectile could possibly go deeper through the thicker hide, thicker muscle, and thicker bones of a large target than it would through those of a smaller target, when the force of impact from a given slinger will be the same in all cases. In short, I can't see any rationale for why the sling projectile would do more damage to the large target than it does to a small/medium one.
I'm inclined to think this is poetic license by Gary to replicate a Biblical David and Goliath situation. But before I house rule this to rectify what I see as a physics error, I am curious to see if anyone out there knows of a solid argument why a sling should do more damage to a large target than it does to a small/medium one. Maybe I'm completely wrong about slings.
Anyone have any thoughts on this?