Eveidently she is hungry when she is hungry. If all parents used measuring cups to ladle their kids food onto their plates at meal time like child care professionals do the kids would be eating a lot less at meal times. Portions for children are much less than adults.
In child care a serving of a veggie, like corn or green beans in 1/4-1/2 cup. Depending on the meal it is part of and if it is the only veggie.
If you are having spaghetti with red sauce then 1/4 cup of green beans is the maximum serving to go with the 1 cup pasta and about 1/4 cup sauce.
It amazes me when I see parents giving their kids the same size servings as they are eating themselves, or they are giving the child an adult size serving and the parent has 2nds and 3rds on their plate for their first servings.
A balanced diet with 3-4 snacks per day plus 3 meals is just that, balanced. The whole food pyramid covers the whole day of food. If they have a grain at snack they do not need another at a different meal. If they have meat at 2 meals they do not need another meat for snack, they can have peanut butter or an egg to supply the protein.
Write down your menu for each day, every little thing that goes in his mouth all day, down to the gum he might chew. Then go to the pyramid and plug it all in to the different areas.
This is PER day, not per meal, PER DAY. Some parents try to feed their kids way too much food and battle them when the kids are truly not hungry. If they eat smaller meals and more often they don't have the ups and downs that long times between meals can cause.
http://pubs.ext.vt.edu/348/348-150/348-150.html
Grains:
3-5 oz per day
1 oz = 1/2 cup of cooked pasta, 1 slice of bread, 1 6" corn or flour tortilla
Veggies:
1-2 cups per day
Veggies measure just as they are, without juice
Salad measures bigger, 1 cup of salad is considered 1/2 cup of veggie
2-3 cups of milk, yogurt, pudding, cheese, etc...
Meat and beans:
2-5 oz
I have posted again and again. In child care we are required to offer food every 3-4 hours. So if I offer breakfast then I better plan on a snack in 3 hours if lunch is not within that 4 hour time frame. Then in 3 more hours they have to have another meal or substantial snack, if they are still there in a couple more hours I have to offer them more food.
Offer your child lots of food throughout the day, they need to eat. They need proper serving sizes so they will eat all they need too.