> The prevalence of dedicated dieting or weight loss efforts has increased over time.

Perhaps, perhaps not. But you'd expect that if there's many different causes making weight gain more common and only one is changes in social pressures (as I've already stipulated in each post).

And, likewise, as stipulated above: overt social stigma is not the only reason why people might prefer to be of a lower weight.

> The success of those efforts has decreased over time.

Well, one reason for this is that we've reduced the use of obviously harmful but effective weight loss techniques like cigarettes and stimulants. The 1950s were the era of the rainbow diet pill clinic, where you'd walk in and get a personalized cocktail of amphetamines and thyroid drugs and laxatives and diuretics to help you control weight.