Diet is more complicated than that. I lost a significant amount by being on a low-carb diet. I didn't really try to control the portion size - though that may have happened naturally. What you eat does matter.
High fat and high protein provides high satiation which prevents the over consumption of calories. I've also lost significant weight on a keto diet. I've also done it with intermittent fasting and eating anything but keeping my calories below my required calorie intake.
