You have to keep in mind that you're looking at two thinking, feeling adults that have capitulated in their struggle to conform to social norms. I know it's hip to pretend that fat people enjoy sweating when they go up a flight of stairs and having to sprinkle Gold Bond in their crevices to forestall dermal yeast infections but fundamentally we're talking about people who have given up on normalcy. They have, as a consequence, adapted to other coping strategies to live their lives to their own best abilities.
They teach their children these strategies not because their behavior is "disgusting and unconscionable" but because it is the culmination of their life experience. They concluded at some point that their children would live as they do, much to their heartbreak, and so they are propagating the wisdom of the tribe.
Mt sister's kids look like a Gap ad. They're impossibly thin. My own daughter will, if she's lucky, be built like Lucy Lawless. She's five but I already know that I will raise her to be her, not my sister's kids.
Don't make me tell the story of the bamboo skewer.