You my friend can try X-plore. It is a file management app which can show you much more details of the file system of your phone. It will allow you to view hidden files as well. Maybe you will be able to find the folder with most weight in it ;)
https://play.google.com/store/apps/details?id=com.lonelycatgames.Xplore
Another method I recommend is using CCleaner Android App. It finds files and cleans your phone intelligently. It is ad free so that is a plus.