The expression "eighteen stone" is mentioned 3 times in the whole book, all in same chapter:
What I dream of is becoming incarnate once for all and irrevocably in the form of some merchant's wife weighing eighteen stone, and of believing all she believes.
You are for ever angry, all you care about is intelligence, but I repeat again that I would give away all this super-stellar life, all the ranks and honors, simply to be transformed into the soul of a merchant's wife weighing eighteen stone and set candles at God's shrine.
Well, this wild legend belongs to our middle ages—not yours, but ours—and no one believes it even among us, except the old ladies of eighteen stone, not your old ladies I mean, but ours.
What is the relevancy of it?