Source : Free On-Line Dictionary of Computing

Parkinson's Law of Data
     
        "Data expands to fill the space available for storage"; buying
        more memory encourages the use of more memory-intensive
        techniques.  It has been observed over the last 10 years that
        the memory usage of evolving systems tends to double roughly
        once every 18 months.  Fortunately, memory density available
        for constant dollars also tends to double about once every 12
        months (see {Moore's Law}); unfortunately, the laws of physics
        guarantee that the latter cannot continue indefinitely.
     
        [{Jargon File}]