To give you short answers to your questions: no, and yes
"Woman" is a noun, a "thing", whereas "big" and "beautiful" are both adjectives, which describe the "thing".
So a woman can be big (or small) or she can be beautiful (or ugly), or as in this case, both big and beautiful. However other women may be small and beautiful, or big and ugly, or even small and ugly.
Now I assume that it is the woman in question herself who self identifies as being big and beautiful. Presumably she does so, because she considers it to be a reasonably noteworthy combination. Like being English and charming, for example
or Irish and honest
So by self identifying as a BBW, she is merely reinforcing the social stereotype that most big women are ugly, or most beautiful women are small.
This is all based on the woman's own self image and others may agree or disagree. However to return to the OP, the only thing we can say "by definition" is that she is a woman.