I think environment has a LOT to do with a person's size...but it's not the only determining factor. There are a lot of thin kids with overweight parents, and they grow up to be healthy people I'm sure. But in many, many cases when you see an overweight child, you'll also see an overweight parent.
One of my best friends growing up was morbidly obese from pre-teen to adulthood. Her parents were also very, very large. And even now I'm noticing the parent/child weight relationship whenever I go places...like the other day I was in the checkout line at a store and there was a family in front of me. The dad was very large, mom was larger. The teenage son was so large he was already having trouble walking (I'm talking 13 or 14 here)...the junior high aged boy was slightly smaller and the elementary aged kid was nearly as large as the middle child. They were stocking up on bags of candy, potato chips, snacks, sodas, etc.
Like I said, I know environment only accounts for PART of who you become...but when the parents eat junk and cook junk, the kids in many cases have no choice but to also become what their parents are.