Big Head Mode (Enemies):
After beating Drek, go to the Goodies screen and perform these moves: 
     Stretch Jump, Flip Back, Flip Back, Flip Back, Stretch Jump, Flip
Back,  Flip  Back, Flip Back, Stretch Jump, Flip Back, Flip Back, Flip
Back and Full Second Crouch.

Big Head Mode (Ratchet):
After beating Drek, go to the Goodies screen and perform these moves: 
     Flip Back, Flip Back, Flip Back, Full Second Crouch, Stretch Jump
and Full Second Glide.