When constructing the batch insert/update/delete command to send to shards, the MongoS write commands scheduler (aka BatchWriteExecutor), accounts for the size of the outgoing request, so it doesn't exceed the 16MB BSON size limit on the way out. However, this doesn't guarantee that the response, which gets generated by the shard will not exceed that size.

This is usually not problematic for ordered=true batches or for smaller ordered=false batches. However for larger ordered=false batches, because of the way the writeErrors is generated, the response can grow to be much larger than the request.

As an example, a batch insert of 100.000 entries which encounters a StaleShardVersion, will result in this kind of situation.