For collections with replicated fast count and size enabled:
Oplog “i”, “u”, and “d” entries have a new root-level object “m” containing a single “sz” element for logical size delta. Count is derived from “i” and “d” entries.

{ "op": ..., "ns": ..., ..., "m":

}

This field may also be present in inner ops of an applyOps if the ops match the conditions above.

Description of Linked Ticket

Original
In the event of recovery, some oplog entries may need to be read to get the correct size and count of collections. These operations are already included in the last checkpoint, so we cannot get the size changes by reapplying them.

We'll need to start recording the size delta of each insert, update, and delete operations in their oplog entries, in a single field object

"m": {
  "sz": <int32>
}

We might want to consider storing the new field in the o2 field for backward compatibility (in testing).