| Why might the poor of our society tend toward obesity at a greater rate than the rich? Is that the question? Aside from the excellent points that Amoeba Protozoa made about average price per calorie, I have a couple of ideas.
The first would be that exercise and outdoor activity have become a luxury for many people. While some working class people still work physically intensive jobs, most blue-collar jobs these days, especially those held by women, don't involve a lot of moving around (think secretary, hairdresser, day care worker, assembly line worker, etc). Add to that the fact that people might be working two jobs to make ends meet, that they have to care for their children on top of that, and that they're probably pretty tired at the end of the day, and there isn't a lot of time, energy, or money left over for step aerobics.
It's interesting to think that at one time, when it was the poor who had to work outside doing heavy labor, it was a sign of status to be overweight and pale: It meant you didn't have to work in the fields. Today, the opposite is true. It is the poor who are trapped behind sales counters, out of the sunlight, never moving, and the rich who can afford personal trainers and tanning booths.
Secondly, eating disorders are epidemic in very poor areas, especially among women. Think about it: Food can give comfort and escape in the same way that alcohol and drugs can, but it costs less and doesn't leave you unable to care for your children or show up on time for work. It's quite a responsible choice in many ways.
Add to that a widespread ignorance about good eating habits, parents too tired to cook proper meals, paychecks too short to buy fresh food and a culture that is shoving food advertisements in your face every thirty seconds, and you have a lot of people who are going to get fatter than they want to be. | 
