In my own understanding, the Higher Self, the Soul, the Divine Spark, of the original Godhead-Divinity is the original microcosm of the Godhead.
Occult knowledge reveals that whenever a God or Godheadic Personality creates emanations of itself, those emanations are generally practically exact replicas. This may be the case here as well, although perhaps to a lesser degree. The Higher Self or immanent Divine Soul is the replica of the Primordial Godhead-Personality (the Anthropos - "(Divine) Human" - of Gnosticism and Manichaeism), although it does also seem to have its owm individuality (perhaps aquired through successive incarnations). Thus the Higher Self, or in Indian metaphysics the purusha - the "little person" or individual Soul, is the image, the microcosm (or "little universe") of the Anthropos or Purusha (capital "P") - the "big person" or Universal Soul, the macrocosm (or "big universe") or Cosmic soul.
This explains why only beings with Souls are microcosms. Only they have the conscious inner Divine presence, and in fact are the vehicle of that inner presence. All the other beings in all the different planes and realms of existence - gods, daimons, devas, spirits, demons, and so on - may have much greater power, but they are not microcosms. They do not reflect the entire Cosmos (macrocosm) in their beings. They only reflect part of it, that aspect of it that they express. They are, as Sri Aurobindo puts it, "typal beings".
So a nature-spirit will express that part of the cosmos that constitutes a particular aspect of biological nature, but it won't express that part of the cosmos represented by the transcendent gods. And vice-versa. Each only expresses that part which it pertains to; none expresses the Whole. Only a being who possesses a Divine Soul, a Higher Self, can express the Whole. It can express it because it is a direct emanation, a direct expression of, the Whole (the Anthropos).