Why does mogodb 3.4.0 uses more oplog space than specified in the configuration file?

Question

In our configuration file of our secondary database we defined an oplog size of 99GB (oplogSizeMB: 99000) but after a few hours we already encounter a size of more than 120 GB (1 file with 74 GB and a second file with 65 GB).

-rw-rw-r-- 1 flugpool fp         16384 Dec 19 10:00 4-4352434068697173865.wt
-rw-rw-r-- 1 flugpool fp         16384 Dec 19 10:00 2-4352434068697173865.wt
-rw-rw-r-- 1 flugpool fp         16384 Dec 19 10:00 0-4352434068697173865.wt
-rw-rw-r-- 1 flugpool fp   69907304448 Dec 19 16:24 10-4352434068697173865.wt
-rw-rw-r-- 1 flugpool fp         36864 Dec 19 17:13 8-4352434068697173865.wt
-rw-rw-r-- 1 flugpool fp   78893129728 Dec 19 17:14 12-4352434068697173865.wt

Is this a bug in version 3.4.0? We did not encounter this problem in mongodb version 3.2.1. or do we missunderstand something about the behaviour of the oplog?

We are running mongodb on SUSE Linux Enterprise Server 11 (x86_64).

systemLog:
   destination: file
   path: "[...]/mongodb.log"
   logAppend: true
   timeStampFormat: ctime
   quiet: true

processManagement:
   pidFilePath: "[...]/mongodb.pid"
   fork: true

operationProfiling:
   slowOpThresholdMs: 10000

net:
   port: 27017
   http:
      enabled: false
      RESTInterfaceEnabled: false

storage:
   dbPath: "[...]/data/db"
   journal:
      enabled: false
   directoryPerDB: true
   engine: wiredTiger
   wiredTiger:
      engineConfig:
         cacheSizeGB: 70
         directoryForIndexes: true
      indexConfig:
         prefixCompression: true

replication:
   oplogSizeMB: 99000
   replSetName: "ANGPOOL_REPLSET"