Not all buildings are drawn at once unless you are in map mode.
get on top of a building (the taller the better) and look around. You will see some fairly large empty spaces where buildings are not being drawn because they are too far away. If you move towards one of those spots, you will see buildings appear, one by one, as you get within the view range.
If you are on a roof and can see large empty spots like this, switch to map mode (right-click the mouse) and you will see all the buildings appear at once, filling those areas.
I took some screenshots to illustrate my point.
In the first image, I am on top of a roof, and you can see some fairly large empty spaces in front of me:
In this next one, I have moved closer to the edge of the building (moving forward from the previous image), and you can see a number of trees and buildings have begun to fill some of the empty area, starting with those closest to me:
In this last image, I have just switched to map mode, so all the buildings are now drawn, and you can really see how empty the two previous images were: