Of course not. If I smoke, for example, I have a higher chance of getting cancer than if don't smoke; if I eat pizza all the day long instead of salad and don't

Of course not. If I smoke, for example, I have a higher chance of getting cancer than if don't smoke; if I eat pizza all the day long instead of salad and don't exercise, I have a higher chance of becoming obese. However I may also never touch a cigarette in my life and still get cancer. I may exercise and eat reasonably well and still be overweight. Some people can smoke 20 a day all their lives and live to a healthy old age; some can eat like pigs and have a size 10 figure. My gut feeling (pardon the pun) is that obesity is mostly caused by the junk that goes into modern food, combined with a more sedentary lifestyle; but that's not to say there aren't a whole lot of other factors - genetics, environment, viral etc - that play a role.