There are several studies showing that obesity is actually HIGHER in households that suffer periodic food scarcity. This makes sense in an environment where caloric density is inversely correlated with price. If you live in a food insecure it would make evolutionary sense for you to a) clean your plate when food is available and b) eat as cheaply as possible -thus consuming a diet rich in Mac and Cheese, Top Roman, the cheapest/fattiest cuts of meat, and the off brand version of frosted sugar bombs.

I think that it is more likely that if there is a plateau that it is because we have maxed out on those kids living in sedentary households. Despite the population level increase in obesity, there are large numbers of health conscious parents who exercise with their children and work to ensure that they eat nutritious meals.