First I created a circle and applied a Particle System to create pine cone style vegetation.
Then I masked the Circle in the Outliner (using the Eye icon), to allow Blender to go faster, and created a little house with a smokey chimney (smoke made with Metaballs) to place in the scene. I converted the Metaballs to Mesh and applied a decimate to it.
Only, when I clicked on the Circle's Eye icon again in the outliner, the Circle was now huge and had undergone a transformation I didn't expect.
The only modifier is the one on the smoke (displace), so I thought that was accidentally applied to the Circle, but I can't see this modifier listed when I select the Circle.
What happened to this Object, and how do I make it look "normal" again in Rendered view?