> If anything, causation is likely to work the other way around, in that larger galaxies have larger central black holes because they have denser central regions.
That contradicts the recent observations which show supermassive black holes were present as early as 700 million years after the Big Bang. That's just impossibly little time for supermassive black holes to have formed if they were just the result of matter in the central part of a galaxy to have "fallen" into the presumably smaller black hole you had in the beginning (presumably, because if you assume the black hole was already big to start with, then you must accept that the amount of matter in the center of the galaxy is a consequence of that black hole being there, not the other way around), which I believe is the theory you're proposing as more likely.
That contradicts the recent observations which show supermassive black holes were present as early as 700 million years after the Big Bang. That's just impossibly little time for supermassive black holes to have formed if they were just the result of matter in the central part of a galaxy to have "fallen" into the presumably smaller black hole you had in the beginning (presumably, because if you assume the black hole was already big to start with, then you must accept that the amount of matter in the center of the galaxy is a consequence of that black hole being there, not the other way around), which I believe is the theory you're proposing as more likely.