My Mom has always been around the size I am now. My DH is fond of saying, "Science has no answer for how much ice cream your Mom can eat [and not gain weight]" She can easily eat a gallon of ice cream laiden with chocolate sauce over the course of a weekend. It's the luck of genetics that lets her do so.
When I was dating, I often surprised my boyfriends by the amount I could eat when they took me out to dinner. I always finished my plate and ate all my dessert. I was asked, "Do you have a hollow leg?" I thought they meant that other girlfriends didn't finish their dinners in order to appear sheik or something. Or that they didn't think a small person could eat as much as I did and not gain weight.
Then, I went on the medicine that put my hunger into overdrive and my eating vastly increased. For the first time, the more I ate, the more I weighed.
I agree with the conculsions of the studies on twins raised apart that our weight is tied to genetics. That you're either thin or not thin, i.e. have the tendency to be heavy. I don't know if thin people process the food they eat differently than not thin people. I don't know if the quantities they eat are different based on different levels of hunger. I didn't see any study datum in the book that discussed these theories.
However, I believe that the environment of fast foods, convience foods, and high fructose corn syrup in just about all packaged goods pushes people from being slightly overweight to being obese and beyond.
And that emotional eating increases the quantity of food consumed which leads to obesity.