As I said before, the availability of food (or lack thereof) to actually feed those populations should keep the numbers from getting out of hand, but still add some dynamism to city size and strength. Cities do grow and prosper, shrink and collapse into ghettos, etc...
Yes, when you don't feed them the population crumbles. I just view the current "max" population as the maximum population you can cram into the space the city occupies. Now obviously there are inconsistencies here, due to the way the maps and cities were originally created, though I like to imagine that some cities are simply happier to have massive poor quarters with people packed in like sardines, while others take a approach that attempts to give the lowly peasants a slightly better life.
Perhaps what could be done is to change population similar to what happened to region stats. 100% is no longer the "standard" for region stats as I understand it, what if there was a lower soft limit that could be considered the "normal" population level, and above that is for when a city is particularly well managed.
