Is the File Cabinet Size (GB) included in the Total Storage Size (MB)?
No. These are two separate components.
What is the difference between File Cabinet Size (GB) and Total Storage Size (MB), when navigating to Setup > Company > View Billing Information > Billable Components?
Total Storage Size (MB) pertains to your data like transactions, contact information of your employees, customers, vendors, partners; sales records like estimates, opportunities, sales orders, sales invoices, purchase orders, etc.
File Cabinet Size (MB) pertains to the files or data you store in Documents > Files > File Cabinet, example of these data are images, customized templates, attachments are business documents.