I've always had a large frame. I played baseball in college at 6'-2" and 195 pounds. After I got married, I gradually became more sedentary and put on weight. At age 45, I got active again, mostly swimming and cycling, and dropped from 295 to 225. Developed back problems, had spine fusion surgery and got sedentary again. At 60, I had a bout with lymphoma that required a stem-cell transplant. The docs warned me that transplant patients often become malnourished because they lose interest in eating, so I should eat as much as I could. I shot back up to 293, and was too weak and tired to exercise for more than a year. Finally, last September, I went on a low-carb diet and started exercising again -- walking, swimming, weightlifting and cycling. I'm now down to 233 and don't plan to stop. My goal is to settle in the 210-215 range, which in my mind won't be bad for a 62-year-old.
So I guess the short answer for why I got so big should be that I had a gland problem -- the gland between my nose and my chin.