The poor is stereotypically painted as haggard and lean and the wealthy CEO (and so forth) as fat and obese, for his very indolence and lack of sluggishness makes him so. Personal counter argument

To arrive at conclusions on any major issue, credible research must be conducted based on scientific, authoritative, empirical evidence. Such, too, must be done in this case and so, inquiring into reasons for the dramatic increase in obesity in America over the last few decades, empirical studies point to factors that include the following: an over-abundance of food availability in America's supermarkets and restaurants, particularly fast-food restaurants (World Health Organization, 2000); the uncontrolled or unreasonable portion-sizes in America's restaurants (ibid); an increase in consumption of sugar-sweetened sodas and sweetend food (Bray, 2004); and an over-abundance of high-fat food choices paired with a lack of palpable low-fat choices all of which may be more accessible to the realtively less-poor than the relatively more moneyed this does not mean that moneyed people do not indulge in these 'opportunities' too. The sedentary lifestyle - due, in part, to advances in technology and transportation, as well as to the appeal of sedentary entertainment options, manifested by (for instance) television, video games and computers - too is attributable to obesity, and, generally, the wealthier you are the more opportunities you have for these activities.

Conclusion

Pollan (2006) opines that the cheapness of American produce is making the poor, rather than the wealthy, obese, for it is the wealthy who can afford the organic products and healthier, but higher priced food that is on the market, whilst the poor turn to that which is closest and most affordable to them, namely...

"Very simple, we subsidize high-fructose corn syrup in this country, but not carrots" (p.100).
But any number of reasons may exist for making people obese, or for people choosing to become obese and, at the end of the day, this is where the crux of the issue lies, people choose to become obese. In short, rather than saying that the poor are obese, and the fat, conversely, are lean, I would be careful to be more precise with my terms and say that the very poor (or the true usage of the term) generally tend to be skinny at best and malnourished at worst (for little to eat), whilst those who are less wealthy than others may tend to indulge themselves in fast food therefore being obsess. This does not exclude wealthy people, though, from not being obese too.

Before we start gnashing our food or matters of corruption that have extended to American nutrition, and blaming obesity on factors out of our control, exhaustive cross-sectional and longitudinal research must be conducted before we come to any definite conclusion. Blaming the obesity of the poor on external factors may not only be a fallacy but may also be a case of shifting the problem to external factors thus freeing humans from responsibility. Identification of causes of obesity should be pursued and steps taken to mitigate the problem. Most importantly, all should be done using authoritative scientific research.

Reference

Bray, G. (2004). The epidemic of obesity. Physiology & Behavior, 82, 115-121.

Pollan. M. (2006) The Omnivore's Dilemma. Penguin: UK

World Health Organization. (2000). Obesity: preventing and managing the global epidemic. Report of a WHO consultation. Geneva, Switzerland: World Health Organization. WHO Technical Report Series, 894.

Wing, R.R., & Polley, B.A. (2001). Obesity. In A. Baum (Ed). Handbook of health psychology (pp. 263-279). NJ: Erlbaum.