Well protein is more filling and you may actually consume less total calories that way. Just guessing.
I remember how, after years of eating high-carb low-fat foods, I discovered how satiating protein-rich meal could be. Well, that and one high in fat. No longer did I feel the constant craving, but at the same time it became easy to go "woohoo!" and eat way more protein than was necessary. I'm sure years from now doctors will ask me what the hell I was doing when they find my kidneys have been wrecked.