The lack of BBW bondage porn is (as others have pointed out) because it's a niche within a niche. Only a reasonably small share of the porn market consumes BBW porn, and only a small share of the market consumes BDSM porn, so logically the amount of people who'd want both at once is going to be a niche within a niche. There's probably bondage porn with larger women in it out there somewhere (virtually _everything_ seems to have at least one video or magazine dedicated to it), but there's a sound financial explanation for why they're so scarce: (perceived or actual) lack of demand.