Agreeing with Worra as I always do on this topic; also TalkinPeace and Running.
The proliferation and sheer appetite for fast food is a huge part of the problem. It's one thing to eat fast food as a meal, why is it even considered as valuable for a snack? Most fast food is just too calorific to eat as a snack and it's not dense enough to keep you going.
I think many of us have just become addicted to the convenience and let's face it, 'taste' of fast food. It's not a coincidence that it's a multi-billion pound industry where considerable resources are dedicated to finding out what makes people buy this product and what can be done to keep them buying it. It's not a million miles away from the tobacco industry in terms of cynically cold hard planning and merchandising BUT, as Worra and TalkinPeace said, we ultimately have control over what we put in our mouths. Understanding the industry is one thing, trying to circumvent it and render that industry 'invisible' is very difficult to do.
I was a kid in the 70s. Money wasn't in great supply and there weren't the convenience foods that there are now. Kids were out and about all day long and after school - there weren't the sedentary hobbies that there are now, kids were out playing. It's very true, I think, that things become a habit very quickly - if it only takes 15 repetitions to make a habit, well that door swings both ways.
I don't know what the answer is; we can't halt these industries overnight and perhaps they have their place but we don't have peer pressure or enough cognisance of what is 'normal' (size, appetite, portions) to make the change so it will just have to come down to personal responsibility. There's no other way.
It worries me that so many people congratulate themselves so wholeheartedly for their fatness, referring to this as 'curves'. It's damaging and giving a massively distorted picture of what is healthy. If you go to countries where overweight isn't celebrated, it's a self-fulfilling prophecy because there are either natural or societal 'brakes' that prevent acceptance of fatness, so people ultimately take control of this themselves. It's tribal. Again, that door swings both ways and the mechanism needs changing in the UK.