After running a migration from a SharePoint Online source to a SharePoint Online destination, you notice that the overall size is larger at the destination than it was at the source when verifying your storage metrics. This is the case even if the files themselves seem to be the same size.
This is expected behavior, due to the way the SharePoint APIs handle the data.
SharePoint will store temporary databases in the background that will increase the total size.
Usually the data is cleared automatically by SharePoint within a few days. If you are close to your storage limit you may want to keep this in mind, and delete unnecessary files pre-migration, increase the storage for the migration, or split up your migration into two parts to give the temporary data a chance to clear out.