I think it looks like a miniature, mainly because of the size of the trees.
Using low poly trees and a particle system, you should be able to fill up your scene.
This might be something to watch: https://www.youtube.com/watch?v=72LPW4S8bns
In another video he managed 20.000.000 trees, now your computer will probably not be able to handle that many, but you should be able to get enough in there.