When migrating permissions, MigrationWiz will report almost exactly double what is displayed in your Destination SharePoint environment because of the way SharePoint reports back data when it applies the migrated permissions.
When going to SharePoint as a Destination, permissions are migrated independently from the files and are only applied after the original file is fully migrated. When the permission is migrated and applied to the corresponding file, SharePoint detects it as a new version of the same file. When this happens, rather than reporting the actual size of the permission that was applied, SharePoint returns the size of the entire file a second time. In turn, MigrationWiz reports the size of the original file again, as if it had been migrated twice.
When you look at the statistics for the items in your project, the document sizes are actually correct. The permissions are the cause for the discrepancy in the total size reported. If they were to be reported correctly, the size of the permissions would be almost negligible when compared to the document sizes. The size of permissions are so small, in fact, that the two ultimately reported sizes appear identical.
There is no workaround at this time, because this is the way SharePoint reports data to us. However, although your data is being reported twice, it is only being migrated once.