I wasn't aware that there were many 500 pounders in society. I think by just estimation, I've seen about 10 people who looked like they weighed that much in my lifetime. Maybe it's an American thing?
My reasoning was that there just isn't enough time to eat and maintain a 500 pound stature. Think about the number of calories that someone needs to consume in order to maintain a weight that high.
Ouch. I can actually eat a large pizza on my own, but after I'm almost throwing up from being so full.
Or for example, last night I went to Mcdonalds and got a double quarter pounder meal, supersized everything, and then went back and got a 2nd burger. Still was feeling like eating after. By the way, I'm only about 155 pounds, 5'10". Besides that, I'm still young and very active so I have a high high metabolism so it's irrelevant. I'm just trying to show that it isn't always about the amount of food you eat.
Are you sure they were 500 pounds? Maybe you don't realize how astronomically large that weight is. Anyways, I'm not going to tell you its genetics because I don't have any proof, I was just going off intuition.