A dirty smelly hut is just as expensive to simulate as a vast mansion.
A mansion will have dozens or hundreds of rooms; how can equally difficult rooms be just as expensive to simulate when there are hundreds more rooms in one scenario than another?
If the em is only in one room at a time, only one room must be simulated in detail at any one time.
Lecture at youtube.
Sorry - haven't watched it yet so no summary, but I expect it to be fun.