My child has become ravenous with food?

I've noticed the last couple of weeks my 8 year old daughter has become so ravenous when she eats. I've never seen her eat with such gusto and determination. She's always been so tiny. She doesn't even register on percentile chart. Now she's thickening up but not growing up with it. I'm getting very concerned. How do I ration what she is eating without killing her confidence. I don't want to wire her for a lifetime of poor eating habits. And poor body image. But she is starting to get a little chunky. And it scares me to see her eat with such love. Any advice? Thanks