Phew, I'm relieved your argument isn't something like "a simulation would by assumption be 'grainier' than a natural universe, and so it would 'split' less often, and so have less 'subjective experience.'"
As to it being a gigantic pain in the ass to simulate an entire universe - sure, and it's unlikely that we're in a simulation. But ignoring units is typically only done when even the exponent is huge, since 10^10^10 meters is 10^(10^10 - 3) kilometers, which is still pretty much 10^10^10. On the other hand, it should only take some well-designed nanotech to keep things running, which is a factor of 10^20 at the worst, which isn't a huge exponent. It's certainly more than we have in our universe, but it's well within what we could have if we had a few extra spatial dimensions or a different history of our vacuum energy or something.
The interesting question is: "do universes exist with a higher computational capacity than ours? How much higher? Orders of magnitude higher? Degrees of infinity higher? Arbitrarily higher? "
I've written a prior post about how I think that the Everett branching factor of reality dominates that of any plausible simulation, whether the latter is run on a Von Neumann machine, on a quantum machine, or on some hybrid; and thus the probability and utility weight that should be assigned to simulations in general is negligible. I also argued that the fact that we live in an apparently quantum-branching world could be construed as weak anthropic evidence for this idea. My prior post was down-modded into oblivion for reasons that are not relevant here (style, etc.) If I were to replace this text you're reading with a version of that idea which was more fully-argued, but still stylistically-neutral (unlike my prior post), would people be interested?