I think big growth for Japan would be no problem if there was advertising, but the question is if the new users would stay.
We had a pretty quick jump from around 90 to more than 110 in Austria, and now we are back to even lower numbers. Most new members quit pretty soon if they realize they get slaughtered in every game. Same would apply for Japan...
I think that kind of summarize it all.
I will have a "wild" guess and say that the size of each BB-nation did not change to much.
Why? Because the uncomptitivness advnatage that the old users has over the new ones.
An advantage that is due to multiple seasons they got "bigger", and is not controled my means of salary cap and luxury cap (for example), which will make the game more competitive.
I'll write again a fact that I've found - three of the largest BB-nations (France, German and Italy) did not had a single team that joined prior of season 4 at last season (which was 17!!!).
And then people are supprised, why this community does not grow...
This game has a lot of possitive things that makes it a good one, but there is no reason to ignore the bugs and bad things this game has which just stand it its way.