We didn't have fat people before we had agriculture. Do you know why that is? Because back then, people had to exercise to get food. Instead, most people can get away with a sedentary life style and eat fatty foods, and then complain when they get fat. If you exercise, diet, or put at least a little bit of effort, you can be skinny too.
Although some may be genetically predisposed to weight gain, some affected by social influences, others by physiological factors. These can all be overcome with effort (Diet and exercise) and in the extreme cases, medication to balance out hormones. Sadly, as a world. We have given way to complacency and sloth. Not wanting to give up our lifestyles and instead making excuses for them. "There is no such thing as big boned, only bone heads"