A weapon’s size category isn’t the same as its size as an object. Instead, a weapon’s size category is keyed to the size of the intended wielder. In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.
Note that this is pretty sketchy given the wide size disparity within these categories, but it's an approximation.
Paizo never bothered to define actual measurements for determining object size, so the above js the closest you'll get to something official.