This became really apparently to me when I went on a "1200" calorie diet for a couple months. It quickly became a 1400 calorie diet because hunger is a bitch. When you start measuring out things like that 3/4c of cereal for breakfast, or counting out each of your 10 chips, or measuring your peanut butter - you realize how much you eat a meal. It also became important for me to differentiate sizes of things, like fruit and eggs. The diet (intentionally) didn't last long, but the things I learned from it stick with me.