What does this mean?? Have a look..

[Netjunkie]

Look at the properties of a folder on my system
What exactly is the difference between 'Size' & 'Size on Disk'?

[sknowonweb]

size => size of bytes on disk
size on disk => clusters consumed by the file ie the cluster allocation.

It means the difference between your 'size' ans 'size on disk' is the wasted space of ur disk .
google for clustors/inodes and will give u better result.

folloe this detailed explanation over here 'http://forums.aliensoup.com/archive/index.php/t-6985'