I'm going to give you a slightly different perspective you might want to think about a bit.
I am a "big" white woman. I have also chosen to be celibate in the final years of my life, because that simplifies everything. I'm happy with my life as it is and I know men (whatever race) have a tendency to look to women to solve their problems. Sorry, I have enough of my own!
See, the my experience has been that the ratio of getting hit on by black men vs. white men is like, 5 to 1. And those men are a hell of a lot younger than I am.
I am not the least bit interested. I've got too many things to achieve in whatever years I have left (I am 73 but I don't look anywhere near that.) Books to write. Photos to take. And the like.
SO may I suggest that the reason you keep getting hit on by big white women is that there's a whole long list of black men who are attracted to that. Why, I haven't the slightest idea. It's especially weird when the guy tops out as an 8 or 9.
There is no such phenomenon among white men. They are mostly (tho not always) stuck on "perfect bodies." So it makes sense that large white women would go looking for black men. They do tend to be far less rigid about female bodies and appreciate a lot of variety in such.
I will not speculate about what black men are thinking in such situations. Only they know (if there's knowledge at all) what the black men/large white women thing is about.
And best wishes with that. I truly hope you'll find what you're looking for.
Have you tried a simple statement saying that you are very flattered by her interest, but you are already taken? (You don't have to explain what "already taken" means.)