Actually, as I understand it, you are actually better off that someone of average or less-than-average body weight. If two people are in a survival situation and have little to no food to eat, the person with more "fat reserves" will most likely be able to survive longer, since their body will have something to use as energy in the absence of food. It is when your body runs out of this fat that you are in real trouble, and someone with little fat to begin with will reach this point much faster.