I think the problem is that maintaining a state on semi-starvation for the rest of one's life is very unpleasant and difficult, and is achieved by very few people:

“Those who doubt the power of basic drives, however, might note that although one can hold one’s breath, this conscious act is soon overcome by the compulsion to breathe,” Dr. Friedman wrote. “The feeling of hunger is intense and, if not as potent as the drive to breathe, is probably no less powerful than the drive to drink when one is thirsty. This is the feeling the obese must resist after they have lost a significant amount of weight.

Controlling for height and sex?

Well, the identical twin parts of the study would automatically control for height and sex :)