There is a tool in your cPanel called Disk Usage that allows you to see the disk space of each directory within your File Manager. This is a very helpful tool to find out where the most disk space is being used.
When you first load Disk Usage, you will see a view of what the disk usage is for a few directories. You are also able to click on the directory name to open a new tab that displays that folder's contents within File Manager.
The bottom of the page will display the entire list of folders that are contained in File Manager. You are able to go through each directory for a more in-depth view. You are also able to click on the folder to take you to the File Manager to see the contents of that directory. You can also sort by disk usage to find out which directories are using the most space.