Parkinson's Law of Data

Parkinson's Law of Data prov.  "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.