As recently as the 1980s movies were overloaded with jokes about fat people, it was extremely common. That's stigma in action culturally.
Answer: literally non-existent.
Indeed, we have the inception of Weight Watchers at the start of the next decade, replacing far crazier things like the cabbage diet being used en masse.
Yes, it was easier in some ways to live a healthier life; one reason why was the rampant social policing of each other's weight.
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.
There was a far more vicious shaming culture 50-70 years ago about things like being obese. And culturally it's still looked down upon to be obese, it's just not as acceptable to be vicious about it (and of course sometimes people still are).
Today however there is a lot more "always on" pressure: social media is a huge component to social, social acceptance, socializing, social learning & sharing, getting to date people, et al. That's a form of individually focused media pressure that didn't exist back then. And sure you can turn it off, not partake, but there are usually serious consequences especially for younger people.
That's the context.