The game always tries to optimize the resources of your device to avoid crashes, hangs, or excessive slowdowns. That is why it always prioritizes the essential graphics from those that are not, and the same happens with sounds, vehicle animations, zoom effects, or background graphics, among others.
What most affects the Simcity BuildIt game on your device (and that the game always keeps a constant eye on) is the screen resolution, the processing power and type of processor, the available internal RAM memory, free space, and so on.
When any of these parameters is not adequate, the graphics may decrease in quality to offer maximum playability.
(*) Contributed by Atlantis