It's true to say calories in, calories out so you can lose weight on pizza. That's true but there's a couple of "real life" issues with it. The first and most obvious one is that if you eat less calorie dense foods, you can eat a lot more of them. If you're trying to stick to 1600 calories then it's gonna be a struggle to not be hungry a lot if you eat all 1600 as a little pile of carbs. And of course hunger is the best way for a diet to fail.

Then the second one I'm a little shakier on but I think there is a difference in GI load of food and how much insulin your food causes your body to release. If I have this right, insulin converts sugars to stored fats if you don't need the sugar right away so there might be a weight loss advantage to triggering less insulin release and having less of your food converted to fat. But what I think I know about this might be incomplete or outdated.