RAW, it's tough without limited wish or better.
Personally, I find it very likely that somebody has created a spell to do this. Something like:
Tailor Magic Arms or Armor
Transmutation
Level: Cleric 3, Sor/Wiz 3
Components: V,S,M
Casting Time: 1 hour
Range: Touch
Target: Unattended magic weapon or magic suit of armor touched
Duration: Instantaneous
Saving Throw: No
Spell Resistance: Yes (object)
This spell changes the size of a single unattended magic weapon or magic suit of armor. It changes the size of the object one category, up or down, as the caster desires. It does not change any other properties of the object.
The spell requires a certain amount of costly oils and magical ingredients. The base cost for material components is one-tenth the base enchantment cost of the object. E.g., a +2 keen flaming dagger has a base cost to enchant of 32,000 gp, so this spell would consume a base of 3,200 gp in magical ingredients.
If the object is formed of costly materials, and the caster is increasing the size, the caster must also pay the difference in the cost of special materials. Conversely, if the caster is shrinking the object, he may apply the difference against the base material cost.
Arcane Material Component: See text