In other words, it is a type of person that is comparative to Big Foot. To better understand:
People say they've seen Big Foot. People say they've seen SSBBWs.
People give vague pictures of Big Foot. People give vague descriptions of SSBBWs.
In both case, neither have been spotted.
However, the SSBBW is more comparable to Big Foot. Not merely because their shoe size are beyond human imagination, but because of their status as a possible species. People say SSBBW exists, but we don't see them. Some claimed they have witnessed them, but could offer no reasonable proof. Those who present a picture usually give something dubious. In the case of Big Foot, the picture is blurred or is merely a shadow. In the case of the SSBBW, the picture is comprise entirely of her stomach.
Truly more research is needed to find these "Super Size Big Beautiful Women".