I always feel really sad (melancholy, not dismissive) when I read the sniping and the superiority stuff about weight on Mumsnet.
I think as a previous poster said, we have serious problems in this country with food technology and home economics. Additive-rich food, lots of convenience food (I'm a consumer of this too I add), and a huge increase in relatively cheap takeaways.
People often follow this with references to the 1950s, and how we've become so much more obese as a nation since. Which I believe is true.
The key thing is that we now generally have two parents working. An excellent and necessary thing in terms of careers/economic productivity/rights. But it introduced another layer of challenge into a household. Some people embrace all of this and still manage to eat healthily. Friends of mine have dads cooking once home. Others are time poor and both parents resort to convenience food.
Then maybe we don't have both parents at home, so perhaps that introduces another layer of things.
There are thousands upon thousands more cars on the roads now than in the 1950s and 1960s. People drive to work, to the supermarket, to the school etc, and can't see the incremental effect this has on weight and fitness.
So my point is, unless we address so many of the things that are changing our society, then we will in general have rising obesity. I'm not including people with thyroid/physical health/MH conditions in this - these are obvious exemptions.