SB 3.28.43

sva-yoniṣu yathā jyotir
 ekaṁ nānā pratīyate
yonīnāṁ guṇa-vaiṣamyāt
 tathātmā prakṛtau sthitaḥ
Synonyms: 
sva-yoniṣu — in forms of wood; yathā — as; jyotiḥ — fire; ekam — one; nānā — differently; pratīyate — is exhibited; yonīnām — of different wombs; guṇa-vaiṣamyāt — from the different conditions of the modes; tathā — so; ātmā — the spirit soul; prakṛtau — in the material nature; sthitaḥ — situated.
Translation: 
As fire is exhibited in different forms of wood, so, under different conditions of the modes of material nature, the pure spirit soul manifests itself in different bodies.
Purport: 

It is to be understood that the body is designated. Prakṛti is an interaction by the three modes of material nature, and according to these modes, someone has a small body and someone has a very large body. For example, the fire in a big piece of wood appears very big, and in a stick the fire appears small. Actually, the quality of fire is the same everywhere, but the manifestation of material nature is such that according to the fuel, the fire appears bigger and smaller. Similarly, the soul in the universal body, although of the same quality, is different from the soul in the smaller body.

The small particles of soul are just like sparks of the larger soul. The greatest soul is the Supersoul, but the Supersoul is quantitatively different from the small soul. The Supersoul is described in the Vedic literature as the supplier of all necessities of the smaller soul (nityo nityānām). One who understands this distinction between the Supersoul and the individual soul is above lamentation and is in a peaceful position. When the smaller soul thinks himself quantitatively as big as the larger soul, he is under the spell of māyā, for that is not his constitutional position. No one can become the greater soul simply by mental speculation.

The smallness or greatness of different souls is described in the Varāha Purāṇa as svāṁśa-vibhinnāṁśa. The svāṁśa soul is the Supreme Personality of Godhead, and the vibhinnāṁśa souls, or small particles, are eternally small particles, as confirmed in Bhagavad-gītā (mamaivāṁśo jīva-loke jīva-bhūtaḥ sanātanaḥ). The small living entities are eternally part and parcel, and therefore it is not possible for them to be quantitatively as great as the Supersoul.