It's not time (very few people who exercise regularly do so to an extent that would undo significant overweight) but more likely a combination of chronic stress and a cultural norm among their family and friends. For younger and more educated people, it's a strong cultural norm to be at a healthy weight, so even people for whom it doesn't come naturally have a very strong social incentive, and they are surrounded by cultural norms that support them. For people in other classes who find it difficult to stay lean, there's much less cultural support (serving sizes, "normal" foods, etc.) and also less social downside to giving up and allowing their weight to drift up.