My daughter is a little over-weight. She is an identical twin and weighs at least 10 lbs more than her twin sister.
She just eats more than either of my other kids, and she always has (even as a baby). My other two kids rarely finish their food, and she is always asking for 2nds (or 3rds).
One summer, I kept juice boxes in the refrigerator. I noticed that she put on a lot of weight. She was about 4 years old, and she had just been getting juice boxes out and drinking them without me knowing it. I moved the juice boxes after that.
I've had to put my foot down with her. I've told her that she can't eat the sweets, and I have to tell her no when she wants 2nds or 3rds. I tell her that she can have as many fruits and vegetables as she wants, but that's it.
It's hard because my son is really skinny, and I'm always trying to get him to eat more.