Replication rate in optimistic mode depends on data set size

From Yammer discussion.

"Data size and resident ratio don't affect default setups (blue, green, red). Replication rate is almost the same.

However optimistic mode is more complicated (cyan, magenta, yellow). The larger data set the higher replication speed. Why is this happening?"

https://www.yammer.com/couchbase.com/#/Threads/show?threadId=324983661