Answer:
It is an arctic double breasted formal wear.
What stays the same size no matter how much they weight?
The person who makes it has no need for it. The person who purchases it does not use it. The person who does use it does not know he or she is. What is it?
Slayer of regrets, old and new, sought by many, found by few.
There is someone, and there is always another, for without the other, there wouldn't be one
You use it between your head and your toes, the more it works the thinner it grows.