I was thinking about the Cat vs. Commoner Conundrum* and moved on to thinking about the size rules in general. From 3.5 on, dealing with non-Medium combatants requires a series of charts and lookups: Str and Dex mods, weapon size, attack and AC bonus, carrying capacity, etc. It seems like it might be preferable to reduce that, particularly for conditions like the Enlarge Person spell that force recalculation on the fly.
The size modifier to attack and AC suggests a way to do that. Characters already take a constant bonus or penalty to attack rolls vs. AC as simple model for smaller creatures having an easier time hitting big ones. Why not roll up the Str and Weapon damage effects into a related system?
Under this system, creatures do not take ability score modifications due to size (including small PC races which lose the -2 to Str). Weapons do not vary in damage die based on size** (a longsword does 1d8, no matter its size). Natural attacks use the dice for Medium creatures (e.g., 1d6 for a bite, 1d4 for a claw).
Creatures of less than Medium size take a penalty to outgoing and incoming damage. This penalty is double the character’s attack/AC size bonus. It is not multiplied on a critical hit; treat it as effectively damage resistance. For example, a Small character (+1 size attack/AC bonus) deals -2 damage on all attacks that require a roll against AC, and takes 2 additional damage from all attacks that target AC. A Tiny creature would have a penalty of 4, a Diminutive creature would have a penalty of 8, and a Fine creature would have a penalty of 16. These penalties can reduce damage below 0.
Creatures of greater than Medium size have a bonus to damage and damage resistance. This bonus is double the character’s attack/AC size bonus. The damage bonus is not multiplied on a critical hit. For example, a Large character (-1 size attack/AC penalty) deals +2 damage on all attacks against AC and increases DR by 2/- vs. attacks that target AC***. A Huge creature would have a bonus of 4, a Gargantuan creature would have a bonus of 8, and a Colossal creature would have a bonus of 16.
These bonuses and penalties should more or less even out the existing modifications for size to physical ability scores and weapon die size. For example, currently Small characters use weapons with around 1 point of average damage less than their Medium counterparts and have a -2 penalty to Str that adds an additional -1 damage in most cases (and cancels out their attack bonus in melee). They probably produce somewhat different results at the greater size categories, but may reduce bookkeeping enough to justify the changes.
* Because of the low granularity of D&D at low levels and the rule about reducing damage below 1, a housecat has a greater than reasonable chance of killing a 1st level commoner in an even fight; the cat is hard to hit and has an innate bonus to hit, dealing 1 HP per hit, while the commoner has a reduced chance to hit the cat.
** Weapons still maintain size designation for purposes of the -2 penalty for using an off-sized weapon. Additionally, characters act as if they were the weapon’s size for purposes of damage (e.g., a Medium character using a small longsword attacks at -2 and deals 1d8-2 base damage. A Medium character using a large longsword must use two hands, takes a -2 penalty, but deals 1d8+2 base damage).
*** It may be necessary to extend this DR to all attacks, to compensate for the removed Con bonus for size. Doing so would require more bookkeeping on the Small characters’ side, however, so it’s probably not worth it.