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...
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.