I noticed in the Tom Hanks film where he was marooned on a desert island that he started off fat and ended up lean.
The lean guys end up dead because their body has high food demands and minimal resources to draw on.
The fat guys have fat to live off of and so they have time to find shelter, start a fire and locate food. Because their body is more efficient at burning food they get more mileage out of any edibles they find.