A common perception of the obese is that they eat far more than the average person. However, studies have not shown this to be true. There are also gender difference involved when studying this.
I found this older study that looked at the issue. For males, lean men actually consumed slightly more calories per day than obese men. For females, the reverse was true: obese females consumed more calories than lean women.
The factor leading to leanness for both men and women was exercise. Lean men and women reported exercising 5-6 days per week, compared to 1-2 days per week for obese men and women.