It depends what you mean by 'numbers'. It sounds like you mean the number of players available to a specific league. USTA is probably more interested in the number of unique individuals playing league tennis, which is not the same thing.
The one thing that really fascinates me about USTA how many different permutations and combinations of leagues there are. To take your 4.0 example - you are right, currently they can play 7.0, 8.0, 9.0 mixed. They can also play 4.0 singles AND 4.0 doubles (potentially also at multiple age levels). Presumably there is also some tri-level in there as well. It seems like (no doubt
@schmke can correct me if I'm off base) a lot of these leagues draw heavily on the same core group of people playing across multiple competitions. No doubt there are a lot of hardcore players out there who enrol in everything, just to get more shots at Sectionals or whatever.
That seems a little bit of a crazy strategy to me. Running leagues is expensive, and there's a lot more margin in USTA memberships than league registration fees, so the marginal benefit of getting existing members to play more often is small. You really don't want to over-cater to your existing membership base - the goal is simply to give them just enough tennis to make them renew their membership each year. The whole game is maximising revenue per match/league - which means more people playing less tennis.
i.e. if improving the quality of the mixed doubles product attracts new people to buy USTA memberships - a reduction in the overall number of mixed leagues/matches isn't a bug, it's a feature.