Here is a gist of the program I used to attempt to insert a 16 MiB document: https://gist.github.com/tfogo/fea1a80c565f7e97f7464c6b0c39640f

Running this results in this output:

ERROR: an inserted document is too large 

This error is due to this line:

https://github.com/mongodb/mongo-go-driver/blob/1aaf676c48ebb378f02f7d93509d5f229c0cede2/x/mongo/driver/batches.go#L11

This line is reducing the space available for documents in a message. The comment implies this is to make space for commands. But messages can be larger than 16 MiB (48 MB by default) so I believe it's incorrect to reserve space for commands that should be used for documents.

I tested reducing reservedCommandBufferBytes to 0. This allows all documents up to 16 MiB in size to be inserted. However, trying to insert a document that is exactly 16 MiB causes another error:

ERROR: (Location40414) BSON field 'insert.documents' is missing but a required field 

This error comes from the server. It looks like the OP_MSG sent to the server could be malformed.