Wait, was that a bug? I always figured it was just based on how insanely difficult it is to keep cities clean as they grow massive. You can still easily hold on to those cities, even very distant ones, by recruiting lots of peasant units to garrison the cities. The security bonus is based on the number of men you have garrisoned and since peasants are the largest units by manpower, they grant the biggest bonus. You wind up with two rows of peasants that are only useful as bait in an actual battle, but give plenty of security bonus to offset the max squalor penalty.
Wait, was that a bug? I always figured it was just based on how insanely difficult it is to keep cities clean as they grow massive. You can still easily hold on to those cities, even very distant ones, by recruiting lots of peasant units to garrison the cities. The security bonus is based on the number of men you have garrisoned and since peasants are the largest units by manpower, they grant the biggest bonus. You wind up with two rows of peasants that are only useful as bait in an actual battle, but give plenty of security bonus to offset the max squalor penalty.