The magician shows to sponge balls, one normal size and one much bigger. He gives the normal sized one to a spectator to hold tightly in his fist, while the big one he squeezes with his fingers trying to make it a little smaller and then closes it in his fist.
After a moment the magician snaps his fingers and opens his hand again: in it he now has the normal sized ball while the big one will incredibly appear in the spectator’s hand!
 
A magical transposition!
 
At the end of the effect if you want, everything could be examined
The normal sized ball has a diameter of 50mm (1,96”), while the large one of 100mm (3,93”)