It's just a matter of advertising.
Look at the proportional search tags in the cloud at the bottom when you do an advanced search. People search for porn in wide terms.
You have the kind of body (especially breasts) that appeal to men who enjoy BBW. And that ranges from Gianna Michaels to much heavier ladies and everything between. So if anyone is looking for pictures of women like you, they don't search for "petite teens", because frankly you are not, and I don't think you can disagree with me. And you have a different, more natural kind of body than what you get in general if you search "busty woman" (just try it yourself, most are silicones).
So BBW, speaking in wide terms (unable to avoid pun), is the closest you can get. Does that mean people think you are obese or fat? No. But it's the most accurate word in porn terminology.
That's my explanation as to why you would be referred to as a BBW: because it's the most rational. You may not think this is right, but that's reality. In internet porn, you're a teen one day and mature the other.
If this bothers you I think you should reconsider posing on the web or try to avoid visiting places where they refer to you as BBW.
I won't refer to you as a BBW anymore, because like DreamSparrow said it is about courtesy and if you don't like it, I don't see any need to call you by terms. Being nice if I can is as natural as your big juicy tits to me, even though I'm not 100% they are, but I assume so
But anyway, that's just me and the bigger part of the Freeones board who reads this, don't expect any significant change from the others.
Hope that was of any help, albeit a bit drawn-out. My personal opinion is that I think you should embrace your BBW image, more men than you may think actually like that a lot and it's usually a sister-word to busty or voluptous.