Take a look around any elementary school playground, then look at old photos of the same playground in the 1970s - kids are FAT these days.
They are FAT and unhealthy.
To answer your question - it is diet. The diet parents feed their kids, combined with an "I am too busy to care" attitude is what encourages kids to be fat.
Added to that, this stupid modern attitude that kids will get hurt if they go out and play. I over hear it a lot, it is all around me. Parents these days are HORRIBLE at being parents.
No, my rant has little to do with the nonsense of a doll - but it does answer your question.