I agree with Elly. Let's not confuse terms here. Plus size is a fashion retail term, and obese is a clinical/medical term. That article is pretty terrible for using size as a medical reference.
That said, I don't think Americans and many other western nations where obesity is the norm have an accurate perception of what obesity is. My mom is considered clinically obese but no one would guess that looking at her. They would just guess she is a bit overweight. After making that realization, she has started a new diet plan and I'm glad for that, but she would not have known if she hadn't researched herself and met with her doctor. She is a size 12/14 (5 feet tall). Would she be obese if she were 5'6? Probably not. My mom is much more petite than most women, so her "normal" is different. It's all relative. That's why using dress size as a gauge for obesity is ridiculous.
I think there is some overlap between plus and missy sizes. You can be a 12, 14, or 16 regular...or a 12, 14, or 16 plus. They are proportioned differently. Some brands that are cut smaller might consider those sizes plus sizes. It isn't really standardized. My mom can't wear plus sizes. She has a large bust but they are just cut too generously all over, especially in the arms and shoulders (big bust, small arms and shoulders...reminds me of someone...).
18 and over is exclusively plus size though. There is no such thing as 18 or 20 missy. I'd say 18 is the official cutoff point, but plenty of brands start at 14 or 16.
