While upgrading your backup drive, you may find that incremental backups that are copied to the new backup location use more space than at the old location.
This can be caused by the copy being performed in a way that does not preserve hard links. This means that each file will use space on the new location for each of the backups stored, whereas on the old location they only use space for each time they were changed.
The copy will need to be performed in a way that preserves hard links. An example of this is below.
rsync -aHAX /old/backup/path/ /new/backup/path/