Disc Defragment

Any time I defragment my hard disc,the space reduces totally,please i need your help and advice.

Welcome
I doubt if the cause is actually defrag, although it may show up then.
Try a disc cleanup.
If that does not work we will clean up your restore points. You will then probably be good again.
http://www.vistax64.com/tutorials/76073-disk-cleanup.html
http://www.mydigitallife.info/how-to-fix-shrinking-disk-space-in-vista/