Wild adventure @Kasir_Barati
Here is something additional about what is considered balanced/triggers the balancer, emphasis added.
https://www.mongodb.com/docs/manual/core/sharding-balancer-administration/#migration-thresholds
Migration Thresholds
To minimize the impact of balancing on the cluster, the balancer only begins balancing after the distribution of data for a sharded collection has reached certain thresholds.
A collection is considered balanced if the difference in data between shards (for that collection) is less than three times the configured range size for the collection. For the default range size of
128MB, two shards must have a data size difference for a given collection of at least384MBfor a migration to occur.