ETA I'd tried to quote the poster asking for a Size 8 at Reiss in LA! Don't know where the quote went!
In my 20s, I remember spotting a dress in DKNY near LA and lifting it to look at it, and the nasty shop assistant walked up to me and said "I don't think you'll find anything that fits you here. We have nothing bigger than an 8, my love."
I was a UK size 8...an American 4. She was very overweight, maybe a UK 18.
I just smiled and didn't respond. As it happened, I decided to try the dress on and went to the fitting room which was manned by other staff members.
She came flying after me...
"That dress is NOT going to fit you. It is TINY."
My Mum was luckily there to step in and said, "if it doesn't fit, it's no big deal is it? My daughter likes the dress and wants to try it. Let her try it."
It fit perfectly, in a size 4. I've never walked out to the main mirror more proudly to "check how it looked".
She was literally apoplectic with rage,
the weirdo... "OH?! It FIT?! What size is that? An 8?!"
She was an utter 🐄. I just said, "Ummm no,
it's a 4. I'm buying it, thanks." Then I asked for the fitting room assistant's name in front of her, "So I can thank the Manager for you helping me today when I pass on my feedback 😂"
Anyway, back to the question...
I've been a 12 at times, I'm 5"5 and it feels just a little bit bigger than I'm happy with but I'd still call it medium. I'd agree 30 years ago it was a slimmer build.
I've got friends who would describe a 12 as fat but they are size 6s and have issues with body image and food. I've heard people infer a size 12 is "too small" but again, they have body image and food issues at the other end of the scale.
I'd say it's a medium sort of size. Slim on some frames, chubby on others but in no way "fat" or "skinny".