If you are referencing to SaaS Backup provided storage, then we won't display it for you as we provided unlimited storage for you (So there is no need for you to be worried about the storage space used).
If you are referencing to BYOS, then you would have created the bucket in your own infrastructure, hence you will see the storage used by that bucket in the respective hyperscalar.
Unfortunately BYOS solution is End of Availability and you will not be able to purchase a BYOS option for SaaS Backup. The only supported storage option going further would be NetApp provided storage (For which we don't show the storage consumed on UI)
If you still want to check the storage consumption, I can provide you the method for calculating the storage :
Identify the total mailbox/OneDrive size of each user and the SharePoint storage metrics for all the sites. You can get the O365 capacity information from report section in O365 admin portal. Following links will help to achieve this
Then add 20% buffer to the above. The buffer always recommend for on-premises sizing, due to MIME conversion bloating (which will cause the size to increase). Say for example, if the customer have 28MB attachment in an email, the Content Length will be 36.6MB once downloaded from O365.
Add the approximate change rate of data per day or month to above. If unsure use 0.5% change rate (1% if conservative). This constitutes the overall capacity.
Now multiply the above with the number of years to get the total capacity.