By numbers, global fertility/population trends and religion are not a "muslim" one, that's a made up western "identity" problem. If anything it's catholic to evangelical sectionism, if you look at Western Africa.

Overall religion is secondary at best, but economically weak tribal/familiy social nucleus cultures vs established nation states that provide reliable social safety not just for relatives.

"It's the Economy, stupid", look at fertility rates of Saudi Arabi, Iran, UAE Poland vs Nigeria.