All boxers:
Win the Bronze, Silver, and Gold belts.
Big heads:
Press X, B, Up, Down when “Press Start” appears at the title screen.
Fight as Mike Tyson:
Win the Gold Belt to unlock Mike Tyson in single player mode.
Flat mode:
Press Down, Up, B, X when “Press Start” appears at the title screen.
Platinum mode:
Press X, B, L, R when “Press Start” appears at the title screen.
Small heads:
Press X, B, Down, Up when “Press Start” appears at the title screen.