Unfortunately, Get-SPODeletedSite -Limit All doesn't provide the actual storage size of the deleted sites. It only shows the resource quota, which is more about the space allocated for the site rather than its actual usage. As you mentioned, this value doesn't reflect the real size of the site, especially for deleted sites.
As of now, there isn't a direct PowerShell command available to retrieve the actual size of a deleted site.
You could go to SharePoint admin center -> Sites -> Deleted sites -> Check used storage of deleted site collection.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.