I too never understood how someone can eat themselves into weighing 500+lbs. I remember way before I started working out I weighed myself and saw 185 on the scale. That's 185 with no muscle to speak of at a height of about 5'10

I looked like Doctor Eggman (big torso, 10" arms and stick legs). My gf at the time said "wow, I never knew you weighed that much". The next day I was on the treadmill 3X/wk. It didn't last, but I can only imagine if I saw 200+lbs on the scale, I would have freaked out enough to actually start working out lol.
I think people who get up too 500/600+ lbs convince themselves that its not that bad when they hit 300lbs or 400lbs because everyone around them weighs that. I was watching a show on TLC a while ago about people getting lypo and cameras would follow them around for updates. And every one of those super obese people had a slightly less obese family member around all the time. So the family members would eat cakes and fast food and shit, while they would have to eat celery and half a cup of yogurt. The thought was "well, im not as fat as you so its ok for me to eat like this". And, of course, the person who got lypo fails in maintaining or losing weight and just get fatter. They compare their weight to that of their family members, so if the entire family is 300lbs, then being slight over 300lbs is considered normal to them.