I'll try to explain it a little bit: files on your hard disk are stored in
clusters. clusters have different sizes depending on
filesystem(fat32/ntfs/...) and HD size. Typical size of cluster is 4kb. So if
you have a 5kb file you will use 2 clusters for it and 8kb(2x 4kb), of which
3kb remains empty and unusable. With many small files this effect may be quite
big. As you are probably windows user, you may use this article for further
reading: <http://support.microsoft.com/kb/140365>