The tip of my hair is thicker at the end

Why is the hair shaft in a piece of hair smaller in diameter than the tip of a hair?

The part of the hair coming out of the scalp is probably miniaturizing; that is why the end of the hair shaft, which is older is thicker.  This  tells you that miniaturization is active in that particular hair’s youngest part.  The same must be true of other hairs on your head,