Many view things in a black and white perspective and make things more complicated than it needs to be. What you might perceive as a BBW may not be to someone else. Simplicity makes life more enjoyable with less stress.
The clothing industry generally uses size 14 and above as BBW, and size 28W and above as SSBBW. But with sizing not standard, and many women different sizes for top and bottoms, this does not always work.