Federal outlays were projected to rise by 6 percent in 2016 —to $3.9 trillion, or 21.2 percent of GDP. That increase is the result of a nearly 7 percent rise in mandatory spending, a 3 percent increase in discretionary outlays (which stem from annual appropriations), and a 14 percent jump in net interest spending.

CBO anticipates that mandatory outlays will be $168 billion higher in 2016 than they were last year. A significant component of that growth is Social Security outlays, which are expected to increase by about $28 billion (or 3 percent)—a percentage increase that is smaller than last year's, primarily because beneficiaries did not receive a cost-of-living adjustment in 2016 but did receive one in 2015. Nevertheless, because the program is so large, even that smaller-than-average increase accounts for one-sixth of the growth in mandatory spending projected for 2016. Federal spending for the major health care programs accounts for a much larger fraction—more than 60 percent—of the projected growth in mandatory spending: Outlays for Medicare (net of premiums and other offsetting receipts), Medicaid, and the Children's Health Insurance Program, plus subsidies for health insurance purchased through exchanges and related spending, are expected to be $104 billion (or 11 percent) higher this year than they were in 2015.