I can tell you how people allow themselves to balloon up to 500, 600, 700 lbs. and so on.
It's called "self-medication". Not too long ago, I was well over 300 lbs. myself and headed for 400, so I can empathize very easily with the super obese. Yes, mental disorders can cause compulsive overeating; once I was treated for mine, the drive to stuff myself until I was miserable went away, and I've lost over 40 lbs without much effort. But many people NEVER discover what, literally, is eating them, and since food is the one addiction that can never be completely overcome due to the necessity of eating to live, they go on until it kills them.
I do know how hard it is to care for such patients. Back when I was a hospital nurse, we had a frequent flyer who weighed ~ 650 lbs. and literally required 10 staff members to get OOB. Bathing him took half a dozen.......three were needed just to hold up the enormous pannus that probably went at least 250 lbs. all by itself. He was one of the nicest patients I've ever cared for; polite, respectful, totally non-demanding, only pushed his call light when he desperately needed something. He passed away at age 48, not surprisingly from CHF. The take-home lesson from taking care of him was simply this: Obese people have ears. They have feelings, too, and they know when we healthcare providers don't like them. Sometimes, they act out because they know they're disliked!
Believe me, your basic 500+ pounder is VERY aware that a) they are dangerously obese, and b) they're not going to live long if they continue eating as much as it takes to maintain such a large body. If you yourself knew you were going to die soon, wouldn't you want to indulge what few pleasures were left to you? I know I would......and since food is often these patients' only source of satisfaction, can anyone out there understand why they may choose to continue stuffing themselves?