Being fat is unhealthy because it does leave you more susceptible to disease. The American health care system wastes billions of dollars each year on diseases and conditions that are a direct cause of obesity. Diabetes, heart disease, high blood pressure, and cardiovascular disease are just a few of the conditions that could be completely avoided by millions of Americans if they just ate/exercised better. It seems that easy access to high calorie junk food and absolutely no exercise (never taking the stairs) is just too much of a temptation for many people... and in evolutionary speak, is a good indicator of that person's genes.
Obesity rarely occurs in nature because obese animals are targeted quickly by predators and quickly dispatched. The fat rabbit cannot escape the lean, muscular leopard chasing after it. In the event of a massive human starvation scenario I would argue that the fat people would be killed off first (possibly by the lean/muscular humans or predators) as the lean humans would require less resources and would be more capable of gathering food for the tribe. What good are obese/severely obese people in the hunter/gatherer world?
Think about all the benefits staying fit yields. It is easier to sit down, get up, row a boat, hunt for food, have sex, run, participate in sports, attract mates (good genes), defend your tribe, etc.
This sounds like a nature vs. nurture discussion.
Nature + Nurture = Who the person grows up to be.
Genes (nature) are the functions but life experiences, environment, etc. (nurture) are the parameters.
GeneX(Twin1, ExperienceA) { return x; }
GeneX(Twin2, ExperienceB) { return y; }
The genes are like restricted scales and within each hold all possible outcomes for an individual. Some of these genes have a greater influence on the end result of an individual than others. Different life experiences, environments, etc. result in shaping each twin differently within each of their genes - since even twins cannot live identical lives and experience the same exact experiences.