Maybe she fills up faster than you think and is hungry.
I never made a rule of eating everything on your plate and no snacks at my house. I don't get why some parents make dinner time a time to enforce discipline or show of power over children.
Why? because I am a grazer and my daughter is just like me. I cannot eat a full plate of food at any sitting. I recall being punished because I couldn't clean my plate. I still harbor tons of resentment toward my parents as well.
We probably have 4-6 very small meals a day which include something like... 1/2 sandwich, fruit, veggie, salad, etc. We snack on fruit, yogurt, nuts and healthy choices.
Example... today, I had a banana and water when I got up, It is 11:15 and I've had 1/2 turkey sandwich and water. This afternoon will be cheese, fruit, etc and dinner is fish. We are not a dessert family at all (just not into sweets) so that is not in the picture at our house.
Moderation is key when you are eating and if you have any junk snack. Sure we have a junk snack every now and then.... we just have that in moderation.