Weight comment, right within the weapons table:
1 Weight figures are for Medium weapons. A Small weapon weighs half as much, and a Large weapon weighs twice as much.
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Weight comment, right within the armor table:1 Weight figures are for armor sized to fit Medium characters. Armor fitted for Small characters weighs half as much, and armor fitted for Large characters weighs twice as much.
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However, as for costs, both weapons and armor rules explicitly state that small and medium variants cost the same (while larger ones get more expensive). This might be an attempt at balancing (though the cost difference quickly becomes neglegible for magical stuff), but remember that material cost is not the only factor that goes into an item's price.
Ummm... did you even read my first quote?
1 Weight figures are for Medium weapons. A Small weapon weighs half as much, and a Large weapon weighs twice as much.
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.