I can see what you're saying OP, obesity has become normalised.
Not having large breasts so dress size is larger, not post baby/surgery body changes, not disability/illness related weight gain etc- most people could see through that.
Also, most socialised people wouldn't comment in the first place to a larger person about weight incase it was something unavoidable.
But it does often seem socially acceptable to tell someone that is slim that they are 'too skinny'. It's unkind and often said by someone who has issues with their own weight an projecting. There may also be health issues behind someone being smaller.
There will already be some larger people who are healthier than their slimmer counterparts. I have been super slim in my twenties and had no muscle definition, little strength and actually didn't look great naked!
I have a friend who is approx a size 26 who is on the go all day, eats well and has no health problems, she also very tall.
In general though, obese people tend to be less healthy. Aside from any illness etc, it can just come down to education around food and exercise at a young age, also positive body image.
If young people only see 'fake bodied' (photoshopped) role models in the model, it may seem unobtainable and push people the other way.
I know as a teen and young adult, I always thought, the skinnier the better. Now in my 30's I'm a little bigger and feel much better in my own skin.
To each their own, as long as your health is the #1 concerns