From what was seen in SERVER-121546, there are a few scenarios where the 16MB catch-all size limit is hit during commit because the size estimator during write staging is incomplete. This ticket should ensure that the size estimator accounts for control block replacements that can change the size of the generated bucket, so that the write path can catch oversized writes before they generate oversized buckets. See the added tests from that ticket on v7.0 for examples and details.