YES... I Wondered The Same Thing. Then, I Thought A Bit More.
The People Out There in The Wasteland Have Little Food and Water.
They Also Run Around All Day and Fight For Survival.
So.. I'm Guessing They Are Just Starving and Exercising and As a Result... Have Little Body Fat?
But Then Again I Think I'm Just Making An Excuse To Solve The Question
Starving? They have TONS of food! All those merchants with a never-ending supply of squirl bits, squirl stew, squirl on a stick, iguana bits, iguana on a stick, mirlurk cakes, noodles, mutfruit. Then you have all the boxes of pre-war food in buildings that haven't decomposed after 200 years or looted, and you have plenty of Wasteland Critters with meat on their bones like Brahmin, Dogs, Molerats, Giant Insects, Mirelurk, etc.
I'd say the Wastelanders eat pretty well, even if the food is all irradiated they still have RadAway to get rid of those pesky rads.