Objects changed by a shrink item spell can be returned to normal composition and size merely by tossing them onto any solid surface or by a word of command from the original caster.
This shows that the item will only resize on a command word or by tossing the item onto a solid surface.
Melee and projectile weapons deal less damage. Other magical properties are not affected by this spell. Any reduced item that leaves the reduced creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown weapons deal their normal damage (projectiles deal damage based on the size of the weapon that fired them).
This shows 2 things. First, that any resizing while being fired would be specifically stated in the spell. Secondly, regardless of item size, the damage is based on the weapon which fires it.
Best Answer
Nope!
Shrink Item is specifically designed for an object, for which creatures do not qualify. You're wanting either Reduce Person or Reduce Creature.
Or Reduce Animal, as the case might be. http://archive.wizards.com/default.asp?x=dnd/lists/spells&tablefilter=reduce