"Humans have ten fingers, Fred has nine fingers, therefore Fred is not a human" but the way we actually think is "Humans have ten fingers, Fred is a human, therefore Fred is a 'nine-fingered human'."

What I actually thought reading this was: "Frodo is a nine-fingered Hobbit"...

In Azkaban is stated that Harry transfiguration of a thin cylindrical layer from the wall is fast because its volume is small. This seems to contradict your assumption.

I think this this is the quote you were looking for.