That's the faces on the rock initially. So if you subdivide 4096 10 times, you're at like 40,960 faces. Waaaaay too much. And that's per rock in your population instance, so if you got a default population at 1000x1000 in size, that's like 372,531,200 faces, just for some rocks. :O That would choke RAM likely on top of the scene if you don't tons of it. Also because these rocks are being computed in TG, I believe they are a bit bigger in RAM than a traditional object from disk.