Computer Movers

Dick and Dirk are employed in a computer hardware store as movers.
One day both of them are asked to move some computers. Dick being energetic that day doesn't feel the computer to be heavy at all. At the same time he sees that Dirk is struggling very hard to lift his computer.

At this Dick says, "What Dirk, my comp has 500 MB HardDisk and yours has just 250, even then you cannot lift it ???"

At this Dirk thinks for a while and replies, "Thats right, but my HardDisk is full and yours is empty"