Learn about portion sizes.
At the top of every food label, you'll find that products serving size. On majority of recipes, you'll find the serving size appropriate for the nutrition facts. Someone may eat relatively clean, nutritious foods but they are eating twice the amount their body needs of those foods. Even healthy foods need to be eaten in balance. Eat until you are not hungry anymore, not until you have to unbutton the top bottom of your pants. Listen to your body. Unlike what many mothers have stated over generations - you DO NOT have to clean your plate. (Just save it for later).
A helpful portion guide:
~ 1 serving of lean protein should fit in the palm of your hand
~ 1 serving of complex carbs from whole grains should fit in one cupped hand
~ 1 serving of complex carbs from fresh fruits/vegetables should fit into two cupped hands together