Zurai said:A sword or axe or mace, etc... those I can see. The main thing there would be to modify the grip and alter the balance of the weapon.
Yeah but a small sized longsword still has less metal than does a medium sized one - so it is more than merely changing the grip and balance for pretty much any weapon.
Small sized weapons weigh less than medium sized ones which weigh less than large sized ones.
from SRD:
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.