Hard drive files are spread across clusters of a given size. The cluster is still used even if it isn't full -- thus the difference.
i don' understand. can u explain a bit more.
if the clusters are used even though they are not full then atleast the size on disc should be 15.4GB.
but the size on disc is only 13.0GB.
i tried google but couldn't find an answer.
No. The file size is the file size. The space used on the disc may be more because the total amount of data takes up more space because of the cluster issue.
In case you're lying awake worrying about this, consider that when you buy a hard drive it's described on the pack as (say) 100gigs but you only get 80 or 90g of disk space. It's to do with the fact that a Megabyte is not 1000 kilobites but 1024.
What matters is that you have enough space for your files and enough space left for the Windows swap file and for defragging when necessary -- thus you don't fill a drive more than about 75% if you can avoid it. In other words, you never get to use all that you've paid for.