What do MongoDB oplogs and banana stands have in common? More than you think.

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.