Children who overeat as kids have twice as many fat cells (large or small) for the rest of their life.

While I accept that this is literally true, I am very skeptical of the connotation -- that greater numbers of fat cells is the mechanism responsible for the difficulty in losing weight and maintaining weight loss.

The hypothesis makes sense, since fat people who lose weight still have a lot of fat cells. But if it were true, one would expect that surgical removal of fat cells would have a significant positive impact on obesity. A couple studies have been done on this issue; I can try to find them if you like. There was not a significant positive impact.

So the inference is that (1) large numbers of fat cells; and (2) difficulty in losing weight and maintaining weight loss, are both the result of a third factor.