Breeding!
Since muslims are supposed to be highly sexed in imitation of the prophet and his companions, it stands to reason that their numbers would grow.
In christianity monogamy is the ideal, and celibacy is also favourable. So we would expect breeding alone to put muslim numbers higher than christian numbers.
However, christianity is still the largest religion.
The Bible is still the best selling book.
Also, Christianity was once the fastest growing religion but reached a plateau.
Islam has yet to reach a plateau, and might need another 600 years to catch up as per age.
Last point, conversions to Islam in the west are often registered at a mosque, like mine was.
I have never heard of anyone registering their apostasy at a mosque - I haven't done it yet.