Slayers has 40+ darn volumes even if the main story volumes are only 13 (if I remember it right) it got expanded for more continuation.
SAO has only half the volumes of Index. If SAO continues on this sale value. I won't be surprised if it manage to touch the sales of Index.
that's why Kamachi can't let go of Mikoto. Look at the vol 03 of new testament. Having her as a cover girl even without much from her boost the sales
even though vol 04 is super crazy and vol 05 is shaping to be awesome it still didn't beat vol 03.
But still my favorite NT is vol 04.