Some spells, such as the Mini Mushroom (Snake Magic), give you a chance to Shrink a monster. 
When a monster is Shrunk, its size is reduced. A mushroom icon and a counter appear in front of it. The counter shows how many times you can draw a card before the Shrinking effect ends.
Smaller monsters, such as the Baby Spider or Frog, are more susceptible to being Shrunk.

Learn more about: Challenge - Defeat X Shrunk monsters