It could also be the problem that many engines have with showing many sprites behind other sprites. Since your sprites seem to have a "keyed" out color, or a color that shows totally transparent, the engine has to redraw many sprites in conjunction with the other transparent sprites behind it. When I say transparent I mean by sprites that have an overlay flag on to show certain parts of the image. So when alot of these sprites/models are behind each other or bunched up, the engine has to iterate through each one, redrawing it, then redrawing whats behind it, I think this is called overdraw. And I might be wrong about this, but due to this overdraw problem, that statue with its many polygons may actually render faster than 2 large sprites that are overlapping each other in the current POV.

And I think your right, I dont think there is too much you can do about this overdraw problem, because many engines have this problem. I havent made a good growth filled level with lush plants and bushes, but I thought about using a smaller scaled version of an LOD system for plants. When a plant reaches a certain distance from the camera, it would morph into its next LOD level, but the resolution of the next LOD would be greatly smaller compared to the last one. I havent tried this out, and I dont know if it would be faster to have many low-res sprites together compared to many high-res sprites together to prevent overdraw.