While playing press Start and enter Square, Circle, Left, Right, Circle, Square.
Press Start and enter Left, Right, Circle, Circle, Circle, Right, Left.
Save the game and return to the main menu. Select the "Resume Game" option, then highlight the previously saved game and press Up, Down, Down, Up, Left, Up, Right, Down.
Pause the game and press Up, Down, Left, Right, Right, Left, Circle.
At the title screen, press Circle, Circle, Circle, Up, Down, Down, Up, Circle, Circle, Circle. Select multi-player mode and choose the Fatboy Mutator. Get killed to have the new Mutator get thinner. Get a kill to have the Mutator get bigger.