Class v. Weight

Here's a post that's probably going to come off as elitist or classist, but is intended as a legitimate question. Is there a correlation between income and obesity?

We keep hearing these dreadful statistics about how overweight Americans are, and I keep looking around and wondering where these people are. Over the past couple weeks I've made several trips to Wal-mart (which I assume has a pretty solidly lower and middle-class clientele), and have consciously noticed how overweight almost everyone seemed. The only connection I could make is that I've spent most of my life in very wealthy places (high school in Park City, Utah; college at Harvard; law school) and neither my peers nor my professors ever approached the average obesity level for our country. Now having spent the summer at the law school, I've also noticed that, for the most part, the only overweight people I see are secretaries or custodial staff.

Has anyone else noticed this? Have there been any studies? Am I totally wrong?