I've never looked at Stella McCartney etc but assume that like most high fashion brands they don't cater to over size 12s.
Its not Stella McCartney, its Adidas, with a Stella McCartney overseen range, but a nicer silkier material. Like Kate Moss for TopShop or George for Asda. Adidas - a very standard manufacturer. Adidas as well - same sizing. Nike - generously sized.
If you are not a genuine size 14 or 16, and don't fit other 14s or 16s, that is a different problem from blaming clothing manufacturers surely?
I have a size 6 skirt and it fits me. That doesn't mean I'm a size 6. It means I have bought one very generously sized skirt labelled size 6. I'm guessing size 16s are much the same as anyone else - some size 16s will be too tight, just as some size 8s are too tight for me.
Given that most women in the UK are a size 14 or over, it seems that many businesses want to cut off their biggest (!) potential customer base in order to preserve their high status, slim women only image.
My guess is that most of the brands mentioned cater for runners, not women over size 14, and that they have done market research which shows that what they produce is what sells profitably. I wouldn't buy from a brand which catered for the average woman instead of the average runner - I want specific running clothing, not useless gym gear which flaps about around your legs if it rains while you're running. The market for runners is huge - all you need to do is go along to your local park run to see lots of women I would say the average of whom is less than a size 14 every single week running competitively. Its a massively growing market as more and more women want to be fit and active. And runners tend to buy multiple sets of sports gear - one for each day of the week would not be unusual. Its a totally different market from the twice-per-week gym session market.