Yeah, I was the same way at 9. Seriously. I could clear out a buffet.
I was 50% smaller then everyone in my class, always. I'm a whole 98 pounds as an adult...pre and post child. I have never weighed 100 pounds until I was pregnant. I can still eat a lot. I have an insane metabolism. People questioned how I could even carry a child.
Unless this child is exhibiting emotional issues or doing badly in school, with friends, and totally withdrawn...actually, it's still really non of your business. Please, stop policing her food. It sounds crazy that you would monitor her food consumption, walk around asking what everyone ate, and counting hot dogs. Butt out, you are being nosy. You said it once, that's enough. This your sister's daughter, and you yourself said she is happy and well cared for. If this becomes a REAL issue, give your sister the credit she deserves, that she is competent enough to deal with it.
ETA: I have never had thyroid, metabolic, or other disorders...just for the record.