as per mention in https://support.box.com/hc/en-us/articles/360043695334-Storage-Usage-Or-File-Count-In-My-Account-Is-Incorrect
That Box Support is able to run a script for specific end-users’ accounts to fix the data inaccuracy issues around user storage size, folder size, and file counts.
Please run the script for my account, as with zero file, it still use 7 Gigabyte storage.
Thanks