It was easier to be skinnier a few decades ago.
This makes immediate sense to me at a surface level. In the nearly 40 years since the 80s, there's been an explosion of food choice, availability, and changes to the type - more meat in the diet, more fast food, more junk food, more processed foods, so on. This should come as no surprise.
But the interesting takeaway here is that the because of all of these dietary choices, many of which were and continue to be more about availability than about conscious decision even though the latter has increased as well, they affect the gut microbiome. And the gut micribiome affects the rest of the body.
It affects brain health. The bacteria in the gut produce approximately 90% of the body's serotonin, used in regulating emotions, and can send signal to the GI system to stimulate or suppress digestion. What you put into your stomach affects your emotion and your stomach can make you crave certain foods, reinforcing the diet that you're feeding it.
Good bacteria in the gut flora feed off fiber; bad bacteria feed off sugar. A few decades ago, and probably trending backwards all through human history, it was easier to be hit and maintain a target weight because the reinforcing feedback loop of gut flora to diet to body health only had a few specific kinds of fuel to choose from, and as the gut flora is changed both by selection in diet and by the changes to the quality of food (which have less mineral content due to intensive farming, for example), makes the feedback loop negative, and harder to break.