Well, that may be true to some extent, but no one is born fat, and the fact is that most obese people got that way through their personal habits, not genetics.
As for not being able to change your body, I would refer you to a detailed biography of Theodore Roosevelt. Read about his early formative years. He was "condemned" to a life of puniness and ill health by everyone except his father, who told the young Roosevelt that he had been born with a healthy mind, but that, "you must make your body."
His brother Elliot (Eleanor's father) was blessed with health early on, but degenerated into a hopeless alcoholic philanderer in adult life.
In other words, you're stuck with your level of intellect, but your body is up to you...