It's all about tradeoffs. In this case, I wonder if there's an "efficient metabolism" gene that makes your body put a higher percentage of incoming nutrients into long-term storage (mostly in fat tissue). Carriers of this gene would be more likely to survive a famine, but less likely to outrun a predator or defend against an attack by another leaner human, who's genes allocate incoming nutrients to be utilized more effectively in the short-term.
Look into Polynesian peoples. They survived long sea voyages, and are known to be generally large people in modern day. Like the guy at my high school whose nickname was "Big Tonga"
