I think it's not really about the raw numbers, as even compensating for height, people can have different body types, different fitness levels, muscle, etc.
I would define the border between BBW and SSBBW more in the terms of how her size affects her everyday life, and how it compares to sizes smaller or bigger.
The effects of her size on the body, what it can do and where it can or cannot fit, would be a better criteria.
I would consider the threshold between BBW and SSBBW to be where one starts getting bigger than most things are designed for. Chairs, cars, seats, booths becoming too small to fit into comfortably (or at all). Certain body positions and movements starts becoming very difficult or impossible (tying shoelaces, walking up more than a single flight of stairs without having to stop, etc.).
I would consider someone a SSBBW from a size where one has to start making significant adjustments in daily routines because things start not being designed for that size. Where going through narrow doorways, fitting inside small cars and behind restaurant booths is still possible, but only barely and with great difficulties. Where every physical activity is still possible, even if only for very short periods of time. A size which starts seriously affecting what one can or cannot do, but still not as big to make most activities impossible.
Here is the rough idea, with the help of pictures:
fantasyfeeder.com/forum/posts
