I.e., the Presumptuous Philosopher is a poor battleground on which to attack SIA, because if used as a heuristic in the past - correctly, not to stake everything on particular theories, but to say that whatever winning theory would make the apparent universe larger - it would have racked up an unbroken string of victories.

I don't think that that's right. Back when it was Big Bang vs Steady State, Steady State lost, even though it implied a larger Universe.

Maybe the ultimate winning theory will hold the Universe to be larger still. But, insofar as there have yet been victories, that one did not go to the one advocating the larger universe.