Don't take my word for it, just look at the CBO's revised 2015 projections. Just look at this wonderful future that awaits us:
In CBO’s projections, federal outlays remain near 21 percent of GDP for the next several years. Later in the coming decade, under current law, growth in outlays would outstrip growth in the economy; outlays would rise to 22 percent of GDP in 2022 and remain at that level through 2025. (Over the past 50 years, outlays have averaged about 20 percent of GDP.) That trend reflects significant growth in mandatory spending—particularly in federal spending for health care, Social Security, and interest payments—offset somewhat by a decline (relative to the size of the economy) in spending subject to annual appropriations.
The government’s interest payments on debt held by the public are projected to rise sharply over the next 10 years because of two factors: rising interest rates and growing federal debt. Because interest rates are now very low by historical standards, net outlays for interest are similar to amounts recorded 15 to 20 years ago, when federal debt was much smaller. As those rates rise, and as debt continues to mount, the government’s cost of financing that debt will climb.
By contrast, discretionary spending is projected to drop from 6.5 percent of GDP this year to 5.1 percent in 2025. That projection incorporates the assumption that the caps on budget authority originally set by the Budget Control Act of 2011, as subsequently reduced, will stay in place through 2021.
https://www.cbo.gov/publication/50724