This illustrates well how boulders impart mass and texture to a landscape. Even though the visible portion of the boulders is relatively small, your mind imagines them much larger. When we see this landscape our mind assumes that much of the boulder is below ground, anchoring the space. We can't see it, but we can feel it.
This landscape is (by design) primarily flat, with just a few key elements providing texture and relief. The stones used are a key part of that.