I would say this:
Look Hon, you appear to be a really nice person, and I wouldn't mind starting a friendship with you, but if my weight is our only similar interest, I really can't see that happening with us.
So, in the future, if you care to discuss any topic under the sun other than my weight, I'll be there for you. We can have a nice little chat and be on our merry way. I am sorry to have to say this, but my weight is my business, and if I want to discuss it, I will put it on the table. Until then, I do not wish to hear another word about it.
