About exercise and wholesome food being privileges: yes, you’re sure right about that. It’s not even the money so much as the time. Cooking good meals, exercising, being rested enough to have the energy for all that, is hugely difficult in this insane world we’ve built. All anyone can do is do what they can, and leave the rest. (And do our bit with our teaspoons to dig away at the mountain of insanity.)

Don’t let the turkeys get you down.

No body type automatically results in clinical obesity, you’re certainly right about that. But short and sturdy is a body type that plenty of people seem to try to fight for no good reason. Also, some body types have a much easier time becoming obese than others. The “80%” (very) round number comes from the extent to which things like longevity, height, and body type are inherited, although I can’t remember where I read the specific reference re body type.

Examples of some discussions in the popular press:

Endocrinologist David Schteingart, M.D., associate director of the General Clinical Research Center at the University of Michigan Health System.

Children whose parents are obese are 10 times more likely to be obese as well.

“All you have to do is look at families and realize that if somebody in the family is overweight, it’s likely the other people in the family are overweight,” Schteingart says. “But if obesity occurs with a familial pattern, is it because of genetics or is it because people share the same habits and the same eating patterns from the word go?
…
Research seems to point to some genetic influence and scientists are now working to determine what genes are involved in weight. Genes have been identified that control how fat is distributed through a person’s body.

“There is evidence also that the way people use their energy is probably genetically determined. There are people who are energy efficient and people who are energy inefficient, and that seems to cluster in families,” Schteingart says.

And another one:

(http://www.snac.ucla.edu/pages/Weight_Fitness/Weight_Managment.htm)

Your genes determine your body type. Some body types carry more fat than others–no matter what you eat or how much you exercise. Percent body fat does not necessarily tell you how healthy or fit you are, or tell you how you look. There are large-framed students who have a higher percentage of body fat who train for triathlons, exercising several times a week and eating a healthy balanced diet. Other students may be ultra-thin and have a lower percentage of body fat, but don’t consume adequate calories or nutrients, lack physical endurance and strength, and don’t look healthy or fit.

One example of recent scientific research (full article behind a fee wall, unfortunately): Functional characterization of human NTRK2 mutations identified in patients with severe early-onset obesity. May 2006. Gray, J., et al. International Journal of Obesity (2007) 31, 359–364.

Search for “weight gain” (with the quotes) and genetics, plus if you’re not a masochist, narrow it down a bit by adding “site:.edu” (without the quotes), and start plowing through the results. You’ll see what I mean.