we are using mongoDB 4.0 on wired tiger engine. For the last couple of days, we are observing that indexSize of primary server(marked in blue in attached file) went up drastically compared to its corresponding secondary servers. As far as our understanding, unlike collection which uses block compression from memory to disk, indexes use prefix compression in memory. Is it something to do with compression or how wired tiger manages space on disk? Could anyone please put some lights as to why sizes are getting so drastically different across members. Attached is the graphical representation of the size with respect to time for last seven days. We could also observe a down word trend now.