Currently the chunk selection in place for the current balancing policy in a sharded cluster is simple and selects the bottom of the shard key range to be moved from the donor shard to the recipient shard. This selection policy can generate sharded collection fragmentation especially when a multiple shards are donating (not concurrently) to the same shard as it's incredibly unlikely the bottom of the shard key range across multiple shards is contiguous. 

With the release of the Auto-Merger in 7.0 we should explore if we can enhance the balancing policy and chunk selection process to maintain range locality while balancing via chunk migrations, enabling the Auto-Merger to squash contiguous chunks together and ultimately keeping the overall chunk count in the cluster minimized.