Ok so after I reset yesterday, my storage space went back to 7.3gb, then within 2 hours it went to 3gb, this morning wake up it goes to 0. Right before I update whatsapp, it went back to 7.1gb, after update whatsapp I'm now at 8.2 gb. Not sure if it will drop again, will report in later..

update: 2 hours passed, dropped 200mb, from 8.2 to 8.0, doesn't seem to solve the issue, maybe it drop much slower now compared to before..