> if some women absolutely can't find something in their size from a specific brand, that makes the brand even more exclusive, like it being "for fit people only".

This is how many brands originally blow up and grow famous. Especially in Asia.

You make clothing in sizes only extremely slim people can wear.

This is an extremely popular brand that specifically does this, and it's hardly the only one:

https://en.wikipedia.org/wiki/Brandy_Melville