Its a combination of all of that. There are more hiding places, more "natural" food to be had, and the all important more bacteria. More bacteria means being able to sustain a higher bioload. This is also contingent on your clean up crew. If you have just fish and nothing to eat their waste then yes you need to keep less fish and do more water changes. But get some starfish, hermits, shrimp, and snails to eat the excess algae and waste and you can keep that extra fish. You must also very carefully mix and match. Different shapes, different species, different colors. Sample good tank would be a sixline, 3 green chromis, 2 percs, goby, and hawkfish/blenny. All look, swim, and just plain behave differently allowing them to coexist without much incident. You could pretty easily do that in a 55 if you wanted to if you also had 75lbs of rock, etc etc.... Bigger tank and you could easily do a tang+dwarf angel......So on and so forth.
Also how often do you want to do water changes. I once heard of a guy who had crazy crazy amounts of fish in a 180 (6 tangs, 3 heniochus, 6 anthias, and more....) but he did water changes all the frikin time. Thats extreme but you get the idea.
