Weight gain in the American population 1960-2000

[Marr]

100. It boils down to portion sizes growing and food becoming more calorie-dense and tastier.

People simply eat too much now-- and it's very easy to do when a simple fast food meal can contain 3/4 or even all of the calories you need for a whole day.

The food industry also pioneered some very clever ways of making more money with larger portions.

There's a very interesting BBC documentary on the topic called The Men Who Made Us Fat.

Sure, we're more sedentary, and being more active has lots of health benefits. But unless you're training intensely for two hours a day (I mean running, swimming hard, etc.), then exercise is going to have very little direct effect on your weight. You can consume all the calories you need for an hour of walking in a couple of onion rings.