The federal budget is shrinking as a percentage of gross domestic product, falling just below 20 percent in the third quarter of 2014. That's down four points from its peak of 24 percent in 2011, according to market analysis firm Strategas' survey of recent Treasury Department data.

"That's a pretty large drop in government spending," said Daniel Clifton, head of policy research for Strategas.

The drop puts current federal spending close to the norm for the last half-century. While the budget has grown in absolute numbers — the omnibus spending bill passed earlier this month totaled more than $1 trillion — federal spending has averaged just over 19 percent of GDP since 1963.

The decline is due to a combination of factors, the main one being the restraints that were put on federal spending in 2011 as a result of the debt ceiling standoff in Congress.

"While the debt-ceiling [fight] got a lot of negative headlines, it actually kept discretionary spending down. Those caps are still in place. They put in additional cuts with the budget sequestration [in 2013]," Clifton said. "There are other drivers too, such as low interest rates."

The recent shrinking is also a function of the recovering economy. Job growth in 2014 was the strongest since 1999 and appears to be accelerating. The Labor Department reported earlier this month that 321,000 jobs were created in November, the best gain in several years.

Overall, the economy grew 3.9 percent in the third quarter, following 4.6 percent growth in the second quarter.

A final factor, Clifton notes, is a decline in health care spending: "The cost of health care has been slowing much faster than the actuaries expected. As a result, Medicare costs have been growing much more slowly than was initially anticipated."