But the point of this diet is that potatoes have a very high satiety so that even if you eat as much you possibly want to you won't eat enough to put on weight.

But the point of this diet is that potatoes have a very high satiety so that even if you eat as much you possibly want to you won't eat enough to put on weight. Rice, with its lower satiety, is much easier to overeat (or eat enough to maintain a higher weight).