But if we saw a thin parent with a big kid, we don't attribute that sort of 'vice' to the parent.
If I see an obese child, I tend to look at the parents, no matter what size and think they are just not feeding their kid properly.

My SDS was obese, his mother rail thin. He ate well but far too much, he was also allowed to eat/drink whatever he liked. 1.5 litres of Coke a day (at the age of 9), 3 vanilla slices for dessert on top of a man sized meal.

He was always puffed out, tired and didn't sleep well. He was always sick and it sucked. It took exH having to talk to the mother very seriously and he took him for an hour walk 3 times a week. It took 2 years but he feels so much better and takes great control over his own diet. He had withdrawal headaches from the coke for a while too...