Shrink Item is a very interesting spell. It enables you to reduce an item to a tiny fraction of its original size, making it easy to transport. It also enables you to bring the item back to its original size via command word or tossing it on a solid surface:
You are able to shrink one non-magical item (if it is within the size limit) to 1/16 of its normal size in each dimension (to about 1/4,000 the original volume and mass). This change effectively reduces the object’s size by four categories. Optionally, you can also change its now shrunken composition to a cloth-like one. 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. Even a burning fire and its fuel can be shrunk by this spell. Restoring the shrunken object to its normal size and composition ends the spell.
One could think about using it for arrows or similar, but this detail on Reduce Person seems to block that usage:
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).
However, this clause there:
"This means that thrown weapons deal their normal damage"
Seems to indicate that, if a throw weapon is resized during flight to its original size, it will deal damage based on that original size, not on the reduced one. This has curios the implication that the momentum of such resized weapons is changed according to the size, increasing alongside the mass and volume of the throw object.
With that in mind,
Suppose that one creates a massive starknife, as a custom magic item, enchanted in such manner that its wielder can activate or deactivate a shrink effect using a swift action. When shrunk, this weapon is around the size of a normal starknife. When expanded, it becomes four thousand times larger.
The idea is that the wielder of this weapon throws this thing and uses a swift action to enlarge it during midfligth, impacting its target as a gigantic steel star.
Is it possible to make this work, or is this too dependent on DM fiat to pull off?
Note: If this is possible to make work with 3.5 material, that would be welcome as an answer too.