Quite simply as your hair recedes & balds more of the skull is exposed. This is not skull expansion this is fewer hairs on the scalp.
Observationally, it appears the skull is expanding, popping out from less hair cover.
But regrow the hair & it will appear the skull is getting smaller.
So inject Botox in the forehead?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5096242
Hope link works. If not google Botox in forehead causes baldness