I think there's probably little doubt that being moderately fat will help you survive longer: that's what body fat is "for", after all, in the sort of teleological view of evolution that Warren Platts might favour. We've got plenty of examples from biology of animals storing up calories against predictable need, and failing to survive if they fail to store calories. There's even some slight evidence from human biology, if you look at the sort of evolutionary "just so" stories that are used in relation to the high incidence of obesity in Polynesians, for instance.
I don't know what to say about the "very fat" of the OP, however. Being "very fat" (in the sense of morbid obesity) is something that evolution hasn't had time to get a handle on, and there's no doubt that it is attended by various metabolic derangements which make it difficult to cope with acute physiological stress.

Grant Hutchison