Weight alone is one of the worst indicators. Even BMI can be misleading, people have different weight distributions.
The effects 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.).
But this question is old, and pops up again and again.
Look here for an example with pictures:
fantasyfeeder.com/forum/posts
