I've watched my 600lb life. I can't watch the whole thing, just disgusts me too much. But I've started and gotten half way, or so, through a few episodes. What I've noticed is that whoever the story is about, their whole family are obese as well, just not as bad as them. I always thought that if someone who's 5'3" reaches 200lbs they'd look in the mirror and say to themselves "ok... well, the rest of my family is around this size, so its not that bad", then they reach 300lbs and think "ok, im a little bigger than everyone else, but its still manageable." But once they reach that 400lb number I would think a reasonable person would say to themselves "ok... this is a bit much..." How they get too 600+lbs still baffles me.
And to the question of how they're able to eat so many calories in a day to get that big. Its not as hard as you imagine if you have a good appetite. I can easily have 3k+ calories of just cashews and macadamia nuts in one sitting and still be able to eat more, I just have to stop myself. I've done it more than once. Now, imagine its not nuts, but other fatty foods, one could easily put away 10k calories in a day if they restrict their protein intake if they ate dawn to night.