I don't believe that's true. The oplog is what powers replication, so it must record every operation altering your data. It's stored in a capped collection, so

I don't believe that's true. The oplog is what powers replication, so it must record every operation altering your data. It's stored in a capped collection, so it stays the same size on disk. If you limit your oplog to only 100MB and have a busy system, you may only store 1 minute worth of operations, but disk size won't be affected. Though, with 100MB your ability to have a useful oplog may be affected and your replica set will likely be unable to replicate fast enough to keep secondaries in-sync.