The ones who get this fat this quickly likely always ate like shit and just had superhuman metabolisms through puberty and just after it and once they age into a normal metabolism they blow up like a hog being fattened for the slaughter. This is also why it's betrayed by arm fat etc. They aren't people who took care of their bodies at any stage. I had a (male) friend like this: as a child and a teen he would eat 4-5 large meals per day and he was still thin and healthy looking. Starting in his mid 20s he quite quickly got fat in the middle and around his face. He's not an inhuman beast or anything, but he has a noticeable gut now and a flabby chin and he's still only in his early 30s.