Big head mode:
At the Main Menu, press Left + Circle, Left + Triangle, Left + Square, Left + X. If you entered the code correctly, you will hear a 'whooshing' sound. Now all non-licensed boxers will have enlarged craniums.

Giant boxer:
Create a boxer in Career Mode named PSIRULES. Save the fighter, then reset your PlayStation. Now load the boxer in Career mode and you'll find his skills and size have increased dramatically...

Fight as a bear:
At the Main Menu, press Right + Square, Right + Triangle, Right + Circle, Right + X. If you entered the code correctly, you will hear a 'whooshing' sound. Then select any boxer to play as a bear.

Regain energy:
When given a choice to hit the heavy or speed bag between fights, hold L1 + L2 + R1 + R2 to regain two to eight points of energy, depending on how long the buttons are held.