Well Totian the skinniest one grew up with a disease that didn't allow him to allow his body to produce needs for his body to grow and evolve. He was taken in by the T.F.F.S and was given the ability but then again it doesn't say it but these character drawings are actually based on the book and comics.

It's okay, the truth is, if I'm interested, worse, I have a doubt if some of them are skinny.

Wow!