Hm, I thought the size limit was a 1 meter. The only reason why its so large is because some of its food (the ghost crystal) is rather large as well. The tasseled phibi is actually the same size as the ghost crystal.
I was kind of dubious about the sudden switch myself. I couldn't find a proper transition, though. I can probably make it a generalist that can replace the cave phibi. Its ancestor is woefully underequipped to navigate the water table anyways.