I haven't read through the whole thread but I was interested that it said 4 was "normal" weight. I think its probably borderline and leaning towards getting overweight. What was the BMI of that model?
As for why people are overweight. That's a pretty complicated question but just my 2c -
1. Education - not enough or too much conflicting information. I often see pretty terrible advice on here too - if you need dietary advice for you or your kids, see a dietician, GP or CHN. Not the internet, forums or fad diet books.
2. It's hard to change bad habits.
3. The supermarkets push bad foods at you with amazing specials. Very often I see chips, soft drinks, biscuits, chocolate, bad cereals etc on special for really good prices. It can be hard to resist. See also point 2.
4. Portion sizes - it's so so easy to overeat and have our kids overeat. So many parents are anxious about their kids eating - how much they eat, what they eat. They load up plates and say "eat it or no dessert" - so kids eat it so they get dessert even if it's too much food. Adults don't need to eat a 300g+ steak several times a week. They don't need to eat carbs with carbs - pasta and bread, potato stew and rice etc. It's so easy to do and many do it.
5. Drinking - as much as I like my wine, alcohol will help you stack on the weight. I think we also need to focus on that bad side of drinking too as a lot of people don't seem to know this.
6. Restaurants and fast food - people eat out of the house much more than ever before. Obviously fast food is bad for you, but a lot of restaurant food is bad for you too - very often high salt and fat and lots of meat and carbs. I bet many people would consider a restaurant meal healthy.
7. Emotional eating - this also starts young. If baby is crying - give them boob/bottle, if a kid is whinging - give them a biscuit, if you've had a crappy day at work - get the girls over for wine and chocolate. People need to find better ways to deal with feelings than food. This is also hard to do and it's also pretty ingrained in our culture. See point 2!
I'm not sure what the answer to obesity is, but I sure hope the next generation is better educated and can make better choices for themselves.