I saw some maps coming by with HUGE cities indeed, so big, with so many items and trees placed that the mission lags
Using fog distance covers alot. What I also usually do (but never posted anything for over 10 years now), is if you want a city to look big, just place some building groups at a further distance then your 'playing area'. Place some tile work between your playing area and the buildings on the distance, some trees and low amount of decorations, and your city will look huge, but in fact it isn't. It's all about what the players see and if you place borders that block your way leaving the area no problem.
I think the best example of this is Diplomatic Immunity. The city looks huge on distance when you fly in towards the UN building, but a lot of the buildings placed in the distance are just for the show.
Also old cod and moh games do this alot with LOW res pictures of buildings on the distance.