You guys really need to understand Lee Know better before overanalyzing everything he says. Most of his self deprecating comments are completely lighthearted
If someone has publicly struggled with their weight, been on strict diets, or shown vulnerability about their body image, calling them any variation of "chubby" or "fat" can be incredibly frustrating. IF IT COMES OUT OF HIS MOUTH ITS FINE BUT OTHERS CALLING HIM THAT IS NOT
The issue with the 뚱냥이 stuff is that we are looking at the human impact of the word, whereas the people defending it are looking at the linguistic intent. Even though the word itself is "endearring" when applied to an animal, doing it to a person completely shifts the dynamic.
The prefix 뚱 is fundamentally rooted in the word for "fat." No matter how much aegyo you wrap around it by adding nyang-i (kitty), you are still explicitly commenting on someone's physical size