The only thing that would normally cause a reduction in size of a differential backup is if a new full backup were run between the differentials. In your case I would check to see if someone ran a full backup between the 25GB differential and the 2GB one. That full would not have to be a LiteSpeed one. Someone may have run a native full backup. You would need to check the msdb tables to be sure. There is a table called backupset that contains the backups.
In order to restore a differential backup you first need to restore it's related full backup.
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David