They simply use "one bald dude" and then just have the engine generate tons of variations from clothing, skin tone, tattoos, piercings and body size.
That gives them freedom to throw waves upon waves of enemies without them looking like a clone army or taking up chunks of memory. "One bald dude is all it takes to make an army of savages," the studio. See the developer video below for more.
Bulletstorm releases on Xbox 360, PS3 and PC February 22nd.