In TO:01:03:11 of the Franz Joseph Star Fleet Technical Manual, all it says is "regulation stretch fabrics". The backstory of this book is that the technical data contained in it was inadvertently transmitted by the Enterprise to the U.S. military during the events of "Tomorrow is Yesterday", and at some points it specifies that '20th Century components have been substituted".
Unfortunately the Technical Manual is non-canon. There has not been any mention of what material the uniforms are made of on screen.
In the animated series episode "The Terratin Incident" the conversation between Spock and Nurse Chaple:
"Yet the uniform on which you wear it fits as well as ever. Uniform made of algae based xenylon, I believe."
The crew encountered comething that shrank organic matter. To eliminate the issue of them running around naked and small, the writers made the uniform composition organic as well.
However, this must have changed by Star Trek: First Contact, as the material released to destroy the Borg aboard the Enterprise-E liquified organic material on contact -- yet Data was bathed in it, and his uniform remained.
Not necessarily. Remember ST:TNG "Rascals"? Wait, I went back and saw a scene from "Rascals" where they had oversized clothes..
>>>Very good. Something else from ST:TAS that should be canon.