So... A woman with smaller breasts ---> smaller cup size---> smaller mask---> more women with smaller breasts die---> more larger-breasted women in the post-apocalyptic world. I smell a conspiracy here.
Size of nose to size of breast ratio is very important in the post apocalyptic world.
In that world, men would have a "decent" excuse to stare at a woman's chest. "Sorry, I was just trying to see if your 'mask' would fit me, if necessary."