http://www.newscientist.com/article/...ce-to-pay.html
The article basically says, in part, that because an increase in testosterone lowers the immune system, survival of the fittest has killed off more muscular guys than skinny guys.
"Testosterone, a hormone that promotes secondary muscle growth, suppresses the immune system of all animals, including people... but the downside of all that brawn is a poor immune system and an increased appetite, a new study finds. Such evolutionary costs could explain why males of our species do not all look like He-Man"
Makes sense I guess!