Hmm, that's a good point. So it would then come down to how much of an expectation of what the simulation is likely to do do you need in order to get away with using fewer qubits. I don't have a good intuition for that, but the fact that BQP is likely to be fairly small compared to all of PSPACE suggests to me that one can't really get that much out of it. But that's a weak argument. Your remark makes me update in favor of simulationism being more plausible.