Why not skip defragging the drive? The drive isn't overly full so the benefit should be minimal. We're not talking FAT32 are we?!?
It would be interesting to see how fragmented the drive is after a week.
Don't know what else Defraggler is doing but it's rather unlikely the space freed up is anything to do with defragging. It must be deleting stuff & maybe that's where the improvement is coming from. Find out what it's doing & do it manually?