In Gears of War 3 for the Xbox 360, you can unlock the Big Head mutator, which will (obviously) give everyone big heads, and has no other negative or positive effects.
To unlock the Big Head mutator, you need to earn the Gold Horder medal by completing 1,000 waves in Horde mode.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.