I don't think there's much recursive improvement yet.

I'd say it's a combination of

A) Before, new model releases were mostly a new base model trained from scratch, with more parameters and more tokens. This takes many Months. Now that RL is used so heavily, you can make infinitely many tweaks to the RL setup, and in just a month get a better model using the same base model.

B) There's more compute online

C) Competition is more fierce.