I think it might have a lot to do with video games, ds type things, laptops, phones with games, the internet, tvs in every room. It seems some parents are happiest when their kids are sitting in front of a screen.
Given the choice most normal kids would be out running around, kicking a ball or climbing the nearest tree - until Santa brings them something with a small screen and suddenly they turn into grunting, pale, grumpy game addicts.
Those things are banned from our house, and there is only one tv. The kids pretty much eat what they want, within reason, but I find after they have been playing outside for a few hours they are more likely to come in looking for a baked potato or bowl of soup than a packet of sweets - it's as though your body really does know what it needs.
As a seperate point, I was the "tubby" child in our house growing up; but not in a morbidly obese way, more that I just had a slightly different body shape and a rounder face than my wiry brothers and sisters. Looking back, I would have just naturally grown into my shape in my teens had I not taken on board a comment my own mother made to me that cut me to the bone (the word "dumpy" was in there.)
So began many years of eating disorder misery, bouts of skeletal-ness and obesity, which I am only now beginning to leave in my past.
So, if you have a child who is slightly larger than his/her brothers and sisters, please be careful what you say to them as you are stepping on seriously dangerous ground. Chances are they know they are the "big" one, so you don't need to point it out to them. Gentle guidance and setting a good example work without anyone getting hurt.