Because, each soul resides in various bodies, has a different personality, karma and existence, associated with it.
- The soul is infinite or vibhu in size.
- The soul is medium sized or madhyakar.
- The soul is very tiny or anu in size.
Let us consider each of these views in detail
The soul as INFINITE in size
But if it is so, then there is no question of going to higher world or lower world after the death. Souls would have existed simultaneously everywhere, at all the times. It would also have bounded the earth, heaven and even hell. Hence there would not have been any need to go and come back from anywhere.
Our SOUL has come out of the SOURCE or GOD, as a part of it.
"Puṇyena puṇya lokaṁ nayati pāpena pāpamubhābhyāmeva manuṣhyalokam"
Hence the soul is not infinite in its size.
The soul as MEDIUM in size
But we see various beings in different sizes ranging from small to giant from the Nature.
For example, if the soul was in the body of an elephant in one life, then its size would be equal to an elephant's body. But in the next life, if it is to take birth as a pigeon, how would such a giant soul fit in a tiny pigeon's body.
Therefore, the size of the soul cannot be finite or medium in size.
The soul as TINY in size
When the consciousness combines with the soul in a physical entity, then it becomes a living thing. If not, then it is a non-living thing. Because the momentum is possible only by the energy given by the consciousness.
"eṣho ‘ṇurātmā" - Muṇḍakopaniṣhad
"aṇupramāṇāt" - Kaṭhopaniṣhad
Hence the soul size is very tiny.
It means, it is able to go everywhere. It can go to the heaven, hell, earth or the God's abode.
It also means that, the soul can access the three forms of time i.e; Past, Present and Future, as it is eternal.