I would be interested in a denominational breakdown too.
I can agree with the reasoning that for many denominations, eating is the only vice left. As a member of such a sect, I think it entirely possible that people who might otherwise self-medicate/comfort with drink/drugs/promiscuous sex/other substances use food instead. Then there are the social aspects–if you can’t go out drinking or to the pool hall, there’s always the ice cream parlor. And churches tend to have big get-togethers where caloric food is involved (my local congregation not so much, but I’ve heard stories of funeral potatoes, and of the spreads the Baptists put on!).
I like chocolate, and dip my own candy every winter. I probably have a fairly high tolerance for chocolate. But I know women at church who leave me in the dirt when it comes to ability to absorb chocolaty substances. They are all slim, though.
OTOH, my own congregation (in N.Ca) seems to be pretty average, weight-wise. The overweight members, by and large, are the ones who have medical problems causing the weight.
I think there are probably several other factors, one being the Southern culture, which historically tends toward rich food anyway. And if the researchers, as noted, were counting religious TV as an indicator of devotion (as opposed to, say, volunteer labor?), I bet that would skew results. I am unconvinced that hours of religious TV exposure = piety.
Why do churches tend to avoid the subject of gluttony these days? Well, an accusation of gluttony from the evidence of obesity is, as we know, often mistaken. There are many underlying reasons for obesity, and many overweight people are more than tired of being told they should just eat less when that doesn’t help. Perhaps preachers are wary of this–they seem to be less judgmental of chronic alcoholism, as well–it’s no longer considered direct evidence of mortal sin. Or maybe some of them enjoy their BBQ a lot too. I would be quite happy to see more emphasis on plain healthy eating and good exercise in the churches, but I can only speak for my own experience, and most of my personal acquaintance are already interested and in good shape.