You will hear JWs coming out with two things that are completely contradictory which they take to both prove the same thing.
1. They have massive growth and that proves that Jehovah is blessing the work they do
2. They are not as large as the Catholic church which proves that they are the 'narrow path' leading to salvation
So, having a lot of followers and also not having a lot of followers are both proof that you are the one and true religion.
Hmmn, Ok ... but there is more.
Think about what they believe regarding #1 above ... that growth is evidence that Jehovah is blessing their work.
How does that work exactly?
God thinks they are doing a sterling job so he does what? Makes people believe? Makes them good? How does free will fit into that?
If that is what god is allowed to do then why can't he just do that for everyone?
Either it makes a mockery of the whole 'free will' thing or he has nothing to do with the growth as he surely cannot influence people's minds if the whole 'free will' test is fair or even necessary.
Of course the only real answer is that the whole belief system is bogus but it's interesting how, if you really think about just one simple part of it, the whole thing just doesn't make sense.