I think the hangover of the war era attitudes to food in a society where calorofic and processed food is easily avaliable has has a lot to do with it, and has distorted society's sense of a healthy portion.
In a (sad) way, I'm quite glad DS1 has food allergies which mean that at 2 1/2 he's never eaten chocolate because of the milk and soya content. I would never have banned it other wise, and it would be for occasional treats, but to not have (mass produced) cakes, chocolate and many other highly processed low nutrition foods in our lives means that he has to have a clean diet. People act like it's a tragedy, and it's not. He's got a great appetite and will eat anything other than salad. On a typical day he'll normally have one snack which if sweet is at least something like a humzinger which has some nutritional value. He is fee to leave food he doesn't want.
Screen time is limited. Left to his own devices, TV/DVDs will dominate his day, but since recovering from pregnancy/ birth of DS2, we've put a timer on the TV and it "breaks" at 10am and won't turn on again until he's in bed. Not being able to switch it on removes the issue.
He walks a lot. I didn't want a double buggy, so most of the time he walks. We have back-up options of a seat on the pram and baby carriers, but they aren't often used as he can easily walk over a mile already. I'm lucky that I have local services about half a mile away a nice distance for us to walk together.
It is very early in his life, but he is off to a decent start on food choices, attitude, family eating round the table and being active, and I hope that is a foundation for a healthy future.
I was away on Guide Camp last week. An ex-brownie was there and when I last saw her 2y ago I thought that she was going down the same path to obesity as her mother. 2y on and I'm sad to say she's developing a large belly. At some point she will either be compromising her health very seriously or have to change many habits that were ingrained early in life which will be tough. I've worked in a wide range of schools, and there is definately a class split with a greater proportion of obese children in poorer areas. In some schools your sense of a healthy weight is very distorted. Slim builds are rare making the overweight look normal and making it hard to judge the boundary of where obesity is, and only those who are on their way to morbidly obese stand out. That is a lot of health problems banking up for the future.
I think it's all a combination of various factors:
lack of time/knowledge/ money/ motivation to prepare fresh foods
vast amounts of retail space dedicated to high calorie/ low nutrition foods
distorted portion size
misleading claims of "health" level of foods
"treats" being a regular component of diet
snacking being treated as normal
less spontaneous active play (physical activity seems to be more structured because of access to safe spaces)
car dependency and often poor access to local services on foot.
Fear of damaging peoples' self esteem about challenging size
a culture of quick fixes for foods and weightloss
high advertising and promotion of unhealthy foods, often on subtle things like special offers and positioning
additives in foods
Prior to DS's allergies I checked labels on foods, and now I'm pretty rigorous and it's amazing how much the "same" food can vary by brand.
Most people gain excess weight subtlely with a small excess being consumed to used, and often small changes can make a big difference over time.