Not sure if Mr. Saletan is aware, but until the mid 20th century, people world-wide had a tendency to regard the fat or plump person as being healthy (notice I said fat and not morbidly obese). In some parts of the world today this is still the general attitude. Why? Because fatter people were/are considered well-fed which equates with being healthy and wealthy. In earlier times (such as the Middle Ages) and in many cultures only the rich could afford to eat the kind of foods and have the lifestyle that encouraged weight gain. Before the 19th Century, some of the medical problems associated with excess weight gain such as diabetes and heart disease generally were not an issue as most people died for other reasons by age 40. The problem with obesity today is less to do with lifestyle but with attitude. Stigmatizing obese people does nothing to encourage lifestyle changes needed for better health, and I think, only increases the likelihood of depression that may discourage change. Unfortunately fatness is not the only characteristic that people today stigmatize. Just ask short people, people with bad acne, bad teeth, big ears or who are not upper class. This includes much of the population.

Lifestyle is precisely what contributes to the nationwide obesity epidemic. Attitude is the response to that. Obviously people had different standards in the past, one of which, as you mention, was life expectancy. Now that we expect our health to carry us further, we have a more accurate conception of the healthiness of fat as it applies to modern life.