That's because the below average sizes aren't all "skinny" if by skinny you mean too thin, unhealthy.
The average UK size is a modern 16. I have vintage clothing 20yrs old. It's mine, it hasn't shrunk or anything. The labels clearly state size UK 10(36). I buy an 8 now to get the same size. Where things are a baggy fit or in cheaper shops where I find the vanity sizing is extreme, I sometimes buy a 6 to get the correct size. My weight is the same all these years, it's the sizes that have changed.
So a modern 16 is a vintage 18/20.
Modern UK16 is large, it's plus size and it's officially the average size. Most people are overweight at that size. That's not fat shaming, it's not an insult. No different than saying someone needs wide fitting shoes. Although that wouldn't necessarily be because their feet were overweight. It's not thinner people's fault if fatter people are oversensitive about their weight or size.
It does people a disservice to pretend that size 16 is fine from a health/weight perspective. Unless you're incredibly tall, it isn't, it's unhealthy weight. Being overweight can cause so many health problems. It's not in anyone's best interest to feed into denial.
Whether a thinner person is unhealthy for other reasons is irrelevant to this discussion, this discussion thread being about clothes size and weight, not other health factors.
Neither does it matter why someone is overweight, it being due to a medical condition or medication doesn't magically make it any more healthy to be that size/weight. Saying something is unhealthy isn't passing a value judgement on somebody's personality, ability to cope, choices, looks, attractiveness or anything else. It's just a fact.