This doesn’t nullify that it could be the Ingush,Chechens and others that are causing the population growth and having bigger families. In fact, if there was a demographic shift (not saying there is), that would also happen.
In fact, fertility rates and Muslims having bigger families has already been written about. http://www.danielpipes.org/blog/2005/08/predicting-a-majority-muslim-russia Not sure if it was in this article by Daniel Pipes but I have read it and admittedly, he is a controversial figure himself.