in terms of energy reserves the fat guys are better of, they done form poisonous products when breaking down proteins for energy
but in terms of survival, the BB's would presumably be able to hike, trap, kill, find water etc.....where as the fat people probably would be left behind...the reason animals dont get obese is they can never fnd that amount of food naturally and not have to work for it
id say the fatties were toast if noone cam looking for them