A little thing called 'reproduction'. End of story.
That would explain part of the growth in the past. Currently JW's have the lowest retention rate (about 20%), which means only 1 out of 5 of those born as JW stay in the religion when they grow up, this according to a study Time (or was it Newsweek?) published some 4 years ago.
I think is a combination of many factors pointed here:
1) There's no significant growth; WT's 'publisher accounting' method is peculiar to say the least.
2) Many fake time (but by doing it they show they consider themselves JW's).
3) Frequently elders make fake reports for those not turning a slip (to look good before the CO).
4) There are people joining the religion (thousands get baptized every year), but this is offset by those leaving.