In one word: GENETIC.
I was overweight when I was born and continued to be throughout my entire life, regardless of what I ate or how hard I exercised. My gastric bypass surgeon measures intestinal length when he does the surgery. Normal small intestines are 19-22 feet long. Mine were 40. "You could eat sand and still gain weight," he told me in the recovery room.
The only drawback to finding out that piece of information is that you have to be opened up to find that out!
I do apologize for my insensitivity.