The incantation that shrinks an object in the Harry Potter universe is "Reducio". This charm is a counter-charm to the Engorgement Charm, which causes objects to enlarge. The first appearance of this spell was in Harry Potter and the Goblet of Fire. It is important to note that like many spells, there must be a mental component to this spell; it is necessary to control the size that you want the object to shrink to, so that size must be controlled mentally. Another incantation that causes an object to shrink is "Diminuendo".