Not necessarily. You could just as easily argue that humans mutate and adapt overtime, which they do, and if muscular-dudes were truly eons more fit for life, we would either mutate or simply die out... And it seems there are more health issues with being huge which could lead to a premature death than that of someone with an average build.

I could also tie in an argument pertaining to the handicapped, but who cares about them anyways?

"Somebody told me when the bomb hits, everybody in a two mile radius will be instantly sublimated, but if you lay face down on the ground for some time, avoiding the residual ripples of heat, you might survive, permanently fucked up and twisted like you're always underwater refracted. But if you do go gas, there's nothing you can do if the air that was once you is mingled and mashed with the kicked up molecules of the enemy's former body. Big-kid-tested, motherf--ker approved."