From what I've seen & read on this board, a lot of the ladies start exercising, eating cleaner & drink tons of water to get healthier, longer hair. I think that is why they seem to be getting slimmer. I can't remember one of the ladies name but she has two pictures in her siggy and a weight tracker and you can clearly see just from her back that she has lost weight and has longer hair.
That's just my two cents.
BUT me personally, my face does look a lot slimmer when my hair is longer; chubbier with shorter hair. That's why I always wore long weaves.
I agree that long hair might make a person look somewhat slimmer because long hair draws attention. When I see people with long hair, I usually pay attention to the color, texture, and to where the length of it stops; the second thing I usually look at is the face so I pay a little attention to the person's body last. But I think that if Monique (the comedian) was to a wig to her booty, she'd still look like a Thick Madame.