SSBBW is an abbreviation for Super Sized Big Beautiful Woman. Ladies in this category generally weigh between 350 (159 kg) and 600 pounds (272 kg). They are larger than BBW and smaller than USSBBW.
Dimensions Magazine defines SSBBW as a woman who is over 350 pounds. This wiki uses the same definition, but adds a separate category for the Ultra-SSBBW who are over 600 pounds.
XHamster says: "There are debates about a girl needing to reach a particular weight number to qualify for this categorization, but in porn scenes it simply denotes a huge body."
See SSBBW for a list of models in this category.