As far as the radical character changes of Teri, Lem and Kenny goes, but especially that of Teri, it's because with Big Mama gone, they all stepped up and became more compassionate, responsible and outspoken members of the family, not just to serve as a good example to the children, but to keep the family together, as well.
Even with all of the soul food that they all eat every week and it giving Big Mama diabetes and eventually killing her, how did everyone else remain healthy, let alone so ridiculously in shape?
Perhaps they only eat like this on Sundays and healthily the rest of the time? Lots of people have a "cheat" day. And just because we don't see them working out doesn't mean that they don't.
