MongoDB Document size too large
The replica set was setup with the internal AWS addresses. So when python connected with MongoReplicaSetClient and the server sent back the addresses it couldn't find the other members of the replica set.

You can update the replica set using rs.conf() to get the config and then rs.reconfig() to set your new configuration.

Or, as I did, add the internal AWS address to the server hosts file, which I was able to do because it was not the internal AWS IP that was used.