Why is MongoDB oplog size different between servers
If you haven't defined in config (file or parameter; replication.oplogSizeMB) what size of opLog you wanted when node was initialized, it is automatically set base on free space on data disk. In wireTiger it's 5% of free disk space (990MB-50GB).

You can change that oplog size if you want, but after that you need to wait until it's full.