That's really silly sweeping statement. Obviously a person's build is affected by their genes, hormones and regulatory systems. We aren't all a uniform height, we don't all have identical frames, our bodies are varied across a spectrum. Some people will tend towards a thinner body and some a fatter one, and that's for all sorts of reasons. The person's relationship with food is a major reason for whether they're fat or thin or in between, but someone's size is not entirely and solely dictated by their upbringing. There are obviously a lot of other factors there - and of course we see body shape and size passed down families and will observe one side of a family might tend towards the stockier and another side the willowy and that children resemble one more than the other.
If we're going to understand why more people are getting fatter, we can't reduce it down to one thing. We've got to acknowledge that it is quite complex. Being fat is not aspirational and most people don't want to be fat or to stay fat if they are. But weight loss is a huge, profitable industry that proves over and over again that it is actually extremely difficult in practice for people to lose weight and keep it off. If you bark 'eat less move more basic science' at these people, it doesn't achieve anything because no one's body is a perfect machine and everyone's lives involve a certain degree of unpredictability. We need to understand how different factors intersect - sleep, stress, coping mechanisms, habits, working conditions, learned behaviour, appetite, hormones, genetics, socioeconomic status, medical conditions - all kinds of things. Sure, the extent to which someone leans on food or not is really significant but if you dismiss everything else going on in their life and in their body then you won't actually be able to get anywhere in fixing the problem.
The reason that's all relevant to the thread is that when we over simplify a complicated situation, not only can we not reach a solution but it also makes it easy to cast blame. If you decide that the only reason a person is overweight is because they just don't have the discipline that a thin person has, you can put all the blame on the individual - and then we get shaming. Fat-shaming usually leads to more weight gain. I know some people think 'if they feel enough shame, they'll lose weight' but it doesn't work like that. You make it harder for people to go to the gym or take up running or any other public exercise because they don't want their body on display. You might push them to comfort eating or, conversely, towards intense restriction in the form of crash diets (which lots of fat people have done many times) that can end up triggering disordered binge eating, out of the body's physical reaction to intense restriction. And dieting long term causes weight gain - that is a proven fact. But fat people are always told to go on diets and do the thing that most likely will make them heavier in the end. If we stay in a shame and blame cycle, I don't know how we manage to break that pattern.
There's an ingrained double standard running through this thread saying - thin people haven't done anything wrong so they don't deserve to be shamed (true!) but fat people are to blame for their bodies and so shaming them is OK. There are some empty words being flung around about 'oh no, shaming isn't OK in any direction' but when those words are both preceded and followed by sentiments that express how fat people are lazy, greedy, undisciplined, bitter and cruel then it shows how meaningless those platitudes are.
If we could accept that there are all sorts of reasons that contribute to weight gain- some of them innate and physical, some of them emotional and learned, some of them to do with external social factors - then maybe we could start to understand that there won't be one simple solution, and then we might be able to work towards something that actually helps.