Yeah, some people burn calories at an easier/faster rate by just living (they have a fast metabolism), but it's a known fact that walking x miles will burn y calories, so you eat proportionally to your caloric expenditure.
Any obese person that argues otherwise is ignoring physics, and is just not trying. My gut feeling (sorry!) is that these obese individuals playing the genetic card are confusing their hunger for their metabolism. It is shown that obese individuals get hungry sooner and that hunger is present longer even after eating (it's a hormone that causes hunger), but this is still preventable.