Aside from the pressure to attract a good Christian husband, there’s also the notion that a slim, healthy body is a better lure to nonbelievers than an overweight one. As Mab Graff Hoover, a sort of Christian Erma Bombeck, put it in “God Even Likes My Pantry” (1983), God wants “us aware that sloppy fat, hanging all over the place (or even well girdled) is not a good Christian witness.”
My own view is that fundamentalist insistence on perfection-through-guilt just fails time and time again. It fails because it relies on shame rather than God's love, and because it inevitably leads to more failures followed by even more strenuous attempts at perfection. This is a moral life governed by external, impossible rules, not internal, gradual grace. Dieting the bad way is an almost perfect analogy to fundamentalism's approach to living morally. I put it this way in The Conservative Soul:
The fundamentalist is always positing an external moral ideal that he must necessarily fail to attain – because he is human. It is outside himself and his job is to internalize it. Fundamentalism as a way of life is therefore a series of ruptures and reforms. It is a cycle of attempts to conform to an external, eternal ideal, and to repeat the process of sin, redemption and sin again indefinitely. In Oakeshott's words, "it has a great capacity to resist change but when that resistance is broken down, what takes place is not change but revolution – rejection and replacement."
Dieting is one of the more secular versions of this cycle. No big surprise that it too is big among fundamentalists – and just as doomed to failure.