My guess is that it isnt entirely just a memory problem. U can load all the cities in there, but more of a city intra-relational problem. Each city entity has to know its surroundings and perhaps be aware of other cities too. the data array or whatever probably reaches peak efficiency at some point which friraxis deemed to be 512 (a number derived from the power of 2, some thought here). So they would proabably have to increase by next order,say 1024?
Anyway, please fraraxis give us the option to increase max cities, hey if it dont work u wont have us chewing your ass some more, hehe