As Joe points out, if JWs measured their membership as Mormons do then they have far more than 9 million members, probably nearer 20 million.
They are doing better than practically all other Christian groups in western countries because most churches are in steep decline. In Scotland hundreds of churches are closing every year, numbers are dwindling, and the average age of attenders is now over 60.
The congregation number has flatlined for about a decade and is down from its all time peak. I wonder if it will start increasing slowly again.