Islam the fastest growing religion?

Is Islam truly the fastest growing religion? If so what is the reason behind it? Is it because Muslims on average are having double the amount of kids then Christians? Or is it because Christians are leaving their religion in vast numbers and joining Islam. Most Muslims claim that many famous Christians are joining Islam, well to that I quote the Bible which couldn't have said it more clearer
Matthew 24 "For false Christs and false prophets will appear and perform great signs and miracles to deceive even the elect—if that were possible."