Heh. I just got a butt plug, of all things, that says "One Size Fits Most" (though I did eventually locate "slender medium" elsewhere on the insert.) That's really only tenuously related to this topic, but I'm medicated, and it found it funny.
I have gained weight due to quitting smoking & thyroid goofiness, but prior to that, yeah... "One size fits most" basically meant "No way in hell." Without the extra weight, I'm petite yet curvy, and clothing manufacturers tend to pick one thing or the other. If you have an ass, you must be the next size up even if that means gaping waist. If you are on the shorter side, you must have a boyish figure. Etc. etc.
Jeans & such are getting better about customization, but lingerie tends to still be either very tiny or very large, in my experience with "fits most."