By what factor does the weight of a melee weapon change if created for a creature with a size class other than Medium?

Question

So it's safe to assume that a Small creature will have a lighter weapon than a Medium creature, and a Large creature will have a heavier weapon than a Medium creature, since the amount of material used in the weapon will differ.

By what factor would the weapon's weight change?

My best guess would be that the weight would be doubled for each size larger, since each size class is effectively twice as large as the previous size class.

Answer 1

So far as I'm aware, Pathfinder details how to determine the cost and weight of weapons designed for Small, Medium, and Large creatures only: Cost is normal for weapons designed for Small and Medium weapons and doubled for weapons designed for Large creatures, and weight is halved for weapons designed for Small creatures, normal for weapons for Medium creatures, and doubled for weapons designed for Large creatures. (See Weapon Qualities on Price and Weight.)

If you're willing to use material from Pathfinder's antecedent Dungeons & Dragons 3.5, one of the last books in that game's line, the Rules Compendium, finally detailed how to compute bigger and littler weapons' costs and weights: For cost, it's a slightly more complicated version of the above:

A larger version costs twice the noted price per size category larger than Medium. A smaller version costs half the price per two size categories smaller than Medium (round up). (152)

But for weight, the original rules are merely extrapolated:

A larger version weighs twice as much per size category larger than Medium. A smaller version weighs half as much per size category smaller than Medium. (ibid.)

I've put this in a chart for convenience:

Size        Cost  Weight
Fine        ×1/4   ×1/16 
Diminutive  ×1/2   ×1/8
Tiny        ×1/2   ×1/4 
Small       ×1     ×1/2
Medium      ×1     ×1 
Large       ×2     ×2
Huge        ×4     ×4 
Gargantuan  ×8     ×8
Colossal    ×16    ×16 

This yields reasonable enough results in both games as almost everything seems to weigh too much anyway.

Answer 2

Weight: This column gives the weight of a Medium version of the weapon. Halve this number for Small weapons and double it for Large weapons. Some weapons have a special weight. See the weapon’s description for details. See FAQ/Errata (at right for information regarding weapons with a Weight of “—” and pricing for crafting them of special materials.

The rules don’t seem to precisely say that you should keep halving/doubling for each size category beyond Small or Large, but this would seem appropriate.