In fact there has been no weight gain at all over the past 30 years in the thinnest quartile of the population -- whatever (poorly understood) factors have caused Americans to weigh more on average now than they did in the 1970s have had very different impacts across the weight spectrum: thin people have gained no weight, people in the middle weigh 10-15 pounds than they did 30 years ago, while the fattest people have gained a lot of weight, which is exactly what one would expect. Furthermore, as even this story manages to note, there's quite a bit of evidence that the trend toward weight gain in the populace in the 1980s and 1990s seems to have plateaued.
The best epidemiological data on the U.S. population is the CDC's National Health and Nutrition Examination Survey (NHANES). This is universally recognized as the gold standard for such surveys, in particular because it's a nationally representative sample that directly measures its participants. NHANES has been ongoing since the 1960s; the most recent data that allows for significant followup is from NHANES III, which was assembled in 1988-1994.
Most recent excess deaths estimates from NHANES III:
Normal weight: 0
Obesity Grade I: -13,865
Obesity Grade II and III: 57,515
Underweight less than 18.5 BMI, normal weight 18.5-24.9, overweight 25-29.9, Obesity Grade I 30-34.9, Obesity Grade II and III 35+ What these numbers mean: In the US population at present, we are seeing about 100,000 fewer deaths per year among "overweight" people than we would if "overweight" people had the same mortality risk as "normal weight" people. Note that the majority of people in the US who according to the government's current classifications weigh too much are in this group. The "overweight" category is to the obesity panic what marijuana use is to the drug war: stories about an "epidemic" of fatness depend crucially on classifying the 35% of the population that's "overweight" as being at some sort of increased health risk. This is simply false, and is known to be false by the researchers who are quoted in stories like the one linked above.
But the situation is much more egregious than even this suggests. Note that the NHANES III data reveals that most people who are classified as obese have a lower mortality risk than so-called normal weight people. About two-thirds of "obese" Americans have a BMI of between 30-34.9, and currently we're seeing about 14,000 fewer deaths per year in this group than would be expected if the group's mortality risk was the same as that of "normal weight" individuals.
Only when one gets to roughly the fattest 10% of the population does the NHANES III data begin to find a relative mortality risk higher than that found among the supposedly "normal weight." And even here, the relative mortality risk results in about three times fewer deaths per capita than observed among the "underweight" (there are approximately four times as many people with BMIs 35+ than there are people with BMIs below 18.5).
The comments cover some of the more common questions-- it's not that a few very sick thin people are making the low weight category look dangerous. There probably aren't enough muscular athletes to completely disconnect BMI from fat percentage.
I can't help wondering if the stats for moderately fat people would be even better (and somewhat better for very fat people) if fat people weren't stigmatized. Not only is there the aggravation of (in many cases) being harassed and insulted, it's harder to make money, and there's a risk of having doctors that blame all one's physical problems on fat so that quite serious medical problems may be neglected.
It's very weird to think that you could be living in a society where the culture could be so comprehensively wrong-headed. The way I got past that threshold was to remember that I'd spent 8 (mostly boring) years in Hebrew school, and it wouldn't have been totally wasted  if I realized that anti-Semitism proved that the majority can be wrong for centuries at a time.
To be fair, a lot of the folk songs and some of the prayers have good tunes.
Link thanks to andrewducker.