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 with a counter appears in front of it, indicating how many cards you can draw before the effect ends. While Shrunk, it takes increased damage.
Tip: Smaller monsters, such as the Baby Spider or Frog, are more susceptible to being Shrunk.

Learn more about: Challenge - Defeat X Shrunk monsters