Hey T. -
As a big, curvy girl with two big, healthy kids (always 90th percentile in height and weight), here's my advice:
Teach her to cook. :)
Obviously, you'd start with having her fix her own snacks -- fun and healthy things like ants on a log (peanut butter on a celery stick with raisins). You might even come up with a little recipe file for her, where you put cards with pictures on them. She can pull out a snack, and assemble the ingredients, and then ask for your help in putting it together (until she can do it on her own). This gives her a creative outlet and teaches her a great skill at the same time. (Can you tell I was a chef once? LOL!)
I let my kids help themselves to snacks as long as they ask me first, and tell me what they are going to have. There are some snacks they may always have, without permission -- fruit and veggies -- which I always have available (and yes, they actually eat it!!)
Lastly, and most importantly -- BANISH ALL JUNK FOOD. Choose nuts, dried fruits, whole grain breads and crackers, lower sugar cereals, low-sugar/low-fat snack bars, lower fat cheese sticks, hummus, peanut butter, sunflower seed butter, lots of different fruits and veggies, low fat yogurt, low-fat/lower-carb tortillas, low-fat ranch dressing.
If there is no junk food in the house -- kids learn to make better choices.
Best,
J.