How do you reduce Vegetation Draw Calls?

I am using the vegetation painter and using the same vegetation pieces over and over again, about 16 pieces in total.

What I do not understand is, how come the engine uses multiple draw calls to keep drawing the same vegetation object as well as shadow objects? Aren't the objects instanced? The more vegetation I put down, even though it is the same 16 objects which should be instanced, the more draw calls the engine makes.

How do I reduce draw calls on instanced objects? Anyone know? There's obviously something here in terms of optimization I am overlooking.

Thanks.