There was some kind of junk-food clinical trial in the wake of "super size me". Unsurprisingly, people were found to react differently and unpredictably to being put on a high-fat, high-calorie diet. This means that your genes are at least partly responsible for your shape. Some people just burn the excess energy off, some of them store it up under their skin and in their liver, which is not good for your chances of impressing members of the opposite (or same if that way inclined) sex with your firm, toned body, or for your liver.
We have unrestricted access to highly palatable and highly nutritious food for the first sustained period in human evolution. It is no surprise that people are fatter than ever, and also no surprise that those who are unhealthily fat (myself included) are so because of a combination of genetic (billions of years of evolving to cope with regular shortages) and environmental (lack of food shortage in the last several decades) factors.