I think diet plays a considerable role, as does the way the "easy lifestyle" is pushed by the advertisers ( I agree with parsleywins points), but so, too, I feel does the lack of exercise. Where once we might have walked to the shops several times a week, now a big shop is done using the car. ( I know, time constraints dictate this). Many children are driven to and from school. Sports and games are replaced by computer games indoors. It's rare to see children out playing anymore - I suppose a lot of this is to do with safety concerns, but it's oh, so sad. We ran off our surplus energy outdoors as children. Yes, lots of children attend clubs, but how do they get there and back?
In consuming larger sized, fattier more sugary foodstuffs without very much excercise between to burn off the calories leads to an increase in size and other health issues. Sugary snacks and drinks seem to be the norm rather than an occasional treat for many, too.
There's no easy solution. Society is so pressured today, but I still feel that time to cook and eat sensibly needs to be factored in - less time dashing here, there and everywhere perhaps? The importance of a balanced diet cannot be underestimated, but I suspect home economics ( as it was called when I went to school) is not considered an important part of the curriculum nowadays. We were taught basic cooking skills, budgeting skills, the importance of all the different vitamins and how to incorporate them into our diet. Not sure that happens very much anymore, though I might be wrong. I might add that I'm not talking too many years ago but then again perhaps the 1970's is!!
I'm not sure how we change any of this, but it's a huge concern.