Mongodb atlas storage issue

My m20 dedicated cluster has around 17gb storage filled by “config” database and around 7 gb storage taken by “local” database…Its too much space taken by them. What is that space ? How can I clear that space taken ?
In config database : system.preimages has taken all of the 16 gb space. I was using mongodb trigger function, but i am in no need of the preimages. How can I free up my space
In local : oplog.rs is taking all 6 gb
Please help me with this