Statistics in the developed world indicate higher incidences of obese / fat people among the poor, the low income earners and the unemployed.
The answer to why poor people are fat can be found from a New York Times article profiling the life of recently layed off lady.
From the article : The confident young woman who had once sold expensive cosmetics had become a middle-aged woman bulging out of sweat pants, her face sagging with exhaustion, her hair matted for lack of access to a shower. Each rejection intensified her fears that she might never work again.
“Is it my age?” she asked. “Is it because I’ve gained weight?” She had been visiting a nearby food bank. “They give us cakes and cookies,” she said. “Then you wonder why poor people are fat! They’re not giving us fruits and vegetables.”