The people in question aren't just "overweight", they're class 3 obese (defined as BMI >40). One way of thinking about the risk level of being that obese is explained here:
Put another way, being that fat is about as damaging to your life expectancy as smoking.Thus, while an 18-year-old white male who is of normal weight and does not smoke can expect to live to 81, the life expectancy of an 18-year-old white male who smokes and is Class 3 obese is only 60 years—a decrease in life expectancy of 21 years, approximately 10 years of which can be attributed to obesity.
The "number of factors" reasoning is mostly a red herring. For almost everyone in these categories, the problem really is just a massive caloric surplus. Sure, there are different ways to wind up with a massive surplus and different mental reasons why someone might do that, but it all comes back to the same core input/output situation.