Not just "vastly immense", but "on a fundamental level, more than exists in our universe, by a factor which is almost certainly greater than zero and whose natural scale is potentially vastly immense".
If you want to argue that the simulation is happening in a different universe, then by that same argument, that is a universe with a lot more stuff going on in it overall than this one, and so the question becomes, why, from an anthropic perspective, aren't we experiencing that one? Which is a weak argument, because if both exist, then SOMEONE would still be experiencing this one, but still, it carries about as much weight as the argument for the existence of that computationally-superior universe, which is basically "you can't prove it doesn't exist".
PS to respond to your edit regarding Kolmogorov complexity:
This is beside the point, because my original argument is not about any possible simulating universe, but about a post-singularity simulation from inside our own universe or one with similar computing power.
Of course it's easy to build something that computes everything computable. It's much harder to build something that computes more than the universe, including some "meta level" capable of referring to the universe but not capable of being referred to by it (where the "simulators" live), but does NOT compute everything computable. The former is uninteresting because it does not increase the measure of any class, and I'd argue that the latter is indeed far more Kolmogorov-complex than just the (simplest members of the class of things isomorphic to the) universe.
I do believe that there's very little if any evidence for a simulated universe. The question is essentially, since we also have little evidence against a simulated universe, what's our prior for the idea?
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?