I always figure the fat is like an emotional protective bubble.
I mean, do you know any adult, who is not disabled that would get praise and YOU GO GORLS! for walking for a few seconds? Or who would make no effort literal slop buckets of food and expect praise , or lose .00001 pounds out of 600 and expect to be looked up at as a role model. the bar isnt low for Amber, it's laying on the ground, covered in shit. No one expects anything because look at her, she's 600 pounds, she CANT do anything.
Methed out fail parents or no, I imagine being so hideously fat and physically incapable molded Amber into what she is now. Every time something was expected of all the other kids, she'd always have that handy excuse of being physically incapable of doing X thing. Choices were always made for her and that's probably exactly the way she liked it, because hey making good life decisions is hard, better let someone else do it. Problem is after the big 18 all that stops and you're on your own. That's when the caregiver quest began, from one to another all the way up to now.