When I went to Paris I noticed that there were no fat people. Not even one. So I asked one of my husbands co-workers, he lives in Paris why this is. Part of it he said is diet, and activity. They walk everywhere. Part of it is genetics. He said something that I thought was pretty smart.
Americans are basically combos of all different races and ethinicities, most of us anyway. Anyway when our ancestors were coming over, sometimes food supplies were short. The people who survived were the ones who had the largest fat stores, to help get them through the winter. That's his theory on why so many americans are genetically prone to being heavy.
I was a heavy kid growing up, not obese, but heavy. When I was growing up, cable was HBO. We only had 2 tvs. Yes, we had Nintendo, but I did play a lot outside. My mom was a SAHM at the time, so there was always good, nutritious food. Snacks were limited to 3 small ones a day.
I agree with everyone, that sometimes it's diet related, but a lot of the time, genetics don't make it any easier.