Most people have a weight gaining plateau. Meaning, for example, they might get up as high as 250 from over eating, but at that point, they don't continue to eat more, they just don't cut down.
Should they cut their food intake in half, their weight would drop. But yo yo dieting causes an upset in metabolism. So if the person at 250, cut calories and dropped to 180, then went off the diet and starting eating as before, they balloon back up. Each time they diet again, they have to take in less and less calories to lose weight.
Now people in the 500+ class, are suffering from more than likely a glandular problem, or a metabolism disorder.
The sad part is that Dr's don't see it early enough to treat it when they are fairly healthy and not seriously obese :(
You left out "they have big bones".
I have also read and watch TV programs where people say they overate in the first place but to maintain the weight because basically it has messed their metabolism up they only have to eat a little over what the body requires and it maintains the weight.