Nycfa:
To me, it’s more of a size/physical appearance rather than a number (such as weight or BMI). I think that’s especially true for women, since very few are alike. By way of example, if you took 5 girls all of the same age, height and weight, you’re very likely to see 5 fairly different bodies. Unlike guys who seem to gain mostly belly, girls seem to gain everywhere or anywhere. I guess that’s why their fabrics are always stretchy, where guys’ usually aren’t. That said, to me, it seems like a height/girth ratio. If a girl is 5’2” tall (62 inches), if her measurement around her widest part is 63 or more, she’s probably an ssbbw
Just my 2 cents…
There's actually a lot more body diversity in male and female bodies. I'm not sure why so many people thing that men only gain one way or women only gain another when there's so much evidence to the contrary.
For example, I knew this guy at work that had the most exaggerated pear shape I ever saw. From his torso up, he looked like he was in the low 200s. But he looked twice as big from the ass down.