I think it's a complicated issue with lots of ignorance and denial involved.
I work with children of juniors and secondary ages. There is a localised class element to this as well. People with more disposable income have better access to formal sporting activities that better balance calories in/ out.
In the secondary age group, I'll see stocky, overweight children. They then stretch out in their growth spurt phase, but when that passes, they then pile on the surpluses. By 15, so many of the girls have heavy, matronly figures and the boys look like rugby players who have passed their peak. I feel sad about it because starting adulthood like that means that they'll have weight as a physical and psychological barrier to a healthier active lifestyle, and it will be a big effort and lifestyle change to lose reach a healthy range. By that point, parental influence is fading and so many of my students drink large daily quanities of sugary drinks.
When I take the DCs swimming, there are increasing numbers of children with multiple rolls of fat. From the toddler classes it starts with the chubby toddler look that some may grow beyond, and they do have parents of similar build which probably skews their perception of normal.
DS2 is of stockier build than DS1,but both have a healthy amount of ribs and vertebrae on display. DS1 looks remarkably like the Peter illustrations from the Peter and Jane books, which looks thin compared to the range of children I see, but both my DCs have consistently held a weight centile in line with their height.
They have large appetites, but snacking isn't encouraged. They will self regulate. Some days they will demolish large portions and other days they'll be more picky. I try to get them out and active for instance today I took advantage of a break in the rain for a short trip to the playground. Their diet is mixed, neither one extreme nor the other. Across the week it balances out.
I think obesity could be considered abuse at the point where a parent wilfully ignores advice on their child's health and their weight continues to deteriorate. There is such a large range of factors at play; routine treats/ snacks, oversized portions, high calorie foods, convenience foods, lack of time, lack of money, access to outdoor space, herd mentality, advertising, pride, sedentary activities, loss of independence, transport...
There is a culture that needs to change. Sadly the change for life messages are corrupted, and schools have too much pressure on their time for a narrow list of accademic priorites. Maybe there needs to be more monitoring between reception and year 6.
All I can do for my children is to continue living a fairly moderate and active lifestyle so that hopefully they'll have a decent template to fall back on when they begin to assert their own choices.