On a large scale, worldwise population goes up by about 1.1% each year.
Last year the JW's had an increase of 1.3%
In that sense they could be said to be outpacing growth of population worldwide.
If you remove the rates of south america and Aftrica however, you find an increase of only about .4%.
So extrapolate what you like there. I would suggest that south america and africa, while lovely continents, also are still largely developing, and are sort of ripe for JW preaching (catholic countries that have experienced grand hypocrisy for example).
That would just be my personal observation of course.