I think there is a complex range of factors at play. For a minority there will be individual health/ development factors, but that doesn't explain a significant change in the numbers of overweight/ obese children as well as the mirroring trend in adulthood.
Life is rushed which affects food choices and activity levels. When I was working F/T, I found myself picking DS1 up from after school club by car because I couldn't afford to lose 5 minutes per day to park up at home en-route and lug a shattered DS2 the 200m walk to school. That 5x5 minutes was 25 minutes per week which was two classes worth of data analysis. I'm pleased to report that since ditching the job we have walked/ biked/ scooted every school run 
Children's activity has become more formalised. DS does a couple of classes per week, but the reality is that much of that 90 minutes is waiting his turn. On paper it sounds good (and he is getting benefits from these sports) but it makes a minimal amount of difference compared to the wider impacts of where he walks from A-B, how he plays at break/ lunchtime, general play at home, playgrounds etc.
Foods have more complex ingredients than they traditionally did, and the full impacts of that are probably unclear. "Healthier" choices are often deceptive.
Snacking is much more prevalent in society. I didn't get in the habit of regular snacks during weaning. The DCs still had access to milk anyway and I wasn't organised enough to lug a load of rice cakes and crudites wherever we went. DS1 regularly comes home with sweets from various sources including things like birthdays at school, much more than I did 30 years ago.
We still have a waste not, want not culture as a legacy of WW2 and days of scarcer food supply. This doesn't combine well with a plentiful supply of calorie dense food. If my DCs have eaten less than usual I tend to ask "is your tummy happy" and remind them that there's nothing else until the next meal. I notice several friends and family pushing at children (and occasionally trying to prompt mine) to eat more and more and turning it into a battle.
We are losing sight of healthy body shapes. My DCs are of a healthy weight, but do look very lean and ribby compared to most. DS1 needs age 5-6 t-shirts for length (he is 6) but he looks like he's falling out of the shoulders, and is still in 3-4 shorts for the waist. I ditched the 2-3s when they began to look like hot pants! He would need several extra inches around the middle to fill age appropriate clothes (although he has slim. These days the illustrations of old books such as Peter and Jane would look thin rather than the normal range. We're told that you can't overfeed babies, so chubby toddlers are no concern. The problem is that around the pre-school age group they should lose that chubbier look and many children don't. They're not obviously overweight, just softer looking until they get to the junior school years. It doesn't take much surplus drip feeding on over time to cause a problem that then becomes apparent at the y6 check. I've taught so many teenagers that huff and puff up the stairs to their classroom because they are unfit and noticeably overweight despite loose fitting uniform. That becomes a barrier to them when they have little confidence at sports, and avoid it as much as possible.
We have lots of euphemisms like "built like a rugby player". Children do have different builds. Some will have wider chests than others, but while a broad built child might look a little like a rugby player in his lean, muscular prime, they shouldn't look like a middle-aged rugby player who's going to seed.
It is hard. There's a lot of time pressure, peer pressure, misinformation and poor opportunities around (thinks of ravenous post-swimming DCs and the crisp/ chocolate laden vending machine...) Some level of it is parental choice (mine aren't always the best), some genes and general health, and a lot of it is down to an obesegenic society, but on a societal level it is a problem, and we are facing an upcoming generation with a significant proportion of avoidable health problems ahead of them.
As a society of consumers, corporations and political standard setting we do need to do more to make positive, healthy choices easier and more accessible.