It takes the six bestselling SFF authors of all time to match Christie's record, and it should be noted that this really includes a huge chunk of real world horror/thriller writing from Messrs King, Koontz, and Crichton. George RR Martin sits in 10th place on that list with less than half of Crichton's sales.
So, despite the exciting surge in mainstream acceptance of SFF seen in the last decade or two, fantasy, particularly of the sword swinging variety, is still a small fish in the book ocean!