How Much Should A 7/8 Yr Old be Eating?

My niece is 7. She is capable of eating 2 servings of pasta and 2 pieces of dinner rolls and will have room to eat some fruit and dessert... When I was 7, I could barely finish a mighty kids or kiddie meal from McDonald's. When she is bored, her first instinct is to eat a meal rather than get a small snack. Her Nana in Texas lets her do what she pleases. I'm starting to worry that she may be eating too much. I try to keep her active, even when she is, the first question is What are we getting for  food. I want 4 cheeseburgers and a medium fry......My stepmom tells her to just eat And not worry if someone doesn't get some food that she had made. She has been here for 2 months and gained 10-15 lbs within that time frame. She eats more than me and I'm 20... How can we break her "boredem eating" habit? I know kids are growing at this age, BUT, I don't think they should be eating more than a grown adult, and I eat a moderate amount of food. Should I even be worried? Hopefully when puberty starts, her body will even out and evenly dispurse. She's a wee bit on the chubby side. But I don't think I should be too concerned with that.I just Don't want her to gain too much and become a size 16 when she's only 10. Then she starts feeling sad and might become bulimic to solve her weight issue.