The cluster balancer calculates the number of documents to move to the destination shard based on the target "balancing unit size" and the average document size for the collection on the source shard. In practice, the amount of data moved can exceed the target "balancing unit size" (if the average document size of the documents being moved exceeds the average document size for the collection).

Currently, an the unbounded amount of data can be moved during a chunk move. This ticket is to add a guardrail to stop (abort) the chunk migration once the "balancing unit size" has been moved.