This is only a guess, but your issue probably is related to the very large model itself. The sheer volume dictates that some parts of the model occur sufficiently distant from the origin as to distort scale. I would suggest breaking the model down into smaller, more manageable elements…each element would comprise a single model, perhaps sized at 1/10th of the original model. You can assemble these smaller component elements into a single master model that would be representative of the overall very large model that you now have, but the components ought to be more easily editable, allowing you to modify entities to consume less volume or memory. If my theory is correct, this should ultimately result in the master model being reduced in terms of memory size as well.
I think it would be interesting to see such a laborious undertaking. If you decide to exhibit any or all of the city model please post the url on this forum.