If we're in a simulation, the computational resources of an object aren't constrained by the physical scale of the object. It's possible that achieving human scale cognition requires brains the size of Jupiter (we don't know because we don't have a full computational accounting of human level cognition).

But in a simulation that could be routed around by having a physical computer the size of Jupiter operate a human scale body in the simulation. You could share and cache compute resources across 6 billion humans within a single human compute core the size of Jupiter (or maybe ~100 Jupiters). Since most computation is probably redundant, you could probably simulate many orders of magnitude more humans with a single human compute core.

To observers within the simulation it would look like human cognition is achievable in 3 lbs of tissue. But this would simply be an illusion created by the simulation environment. There'd be no way of knowing for sure until we fully reverse engineer a single human brain down to the level of base compute operations.