The Chart above shows the spending of the US Government as a percent of GDP over the past 110 years. Government spending became a larger part of our economy for numerous reasons.  A large part of the increase is due to the addition of Social Security, medicare, and medicaid. And part of the increase is due to the need for a ready to fight military..  


Growth relative to GDP actually leveled out and actually dropped a little in the 1990's.  The recent and troubling spike recently is due to the very poorly economy, large stimulus, and an aging population.


http://www.usgovernmentspending.com/us_20th_century_chart.html

http://en.wikipedia.org/wiki/Government_spending