In the Reagan buildup of the early-1980s, the relationship between the size of the budget and the size of the force began to change. While the budget grew by 78 percent in real terms from the trough of FY 1975 to the peak of FY 1985, much of the increase was used to modernize DoD’s inventory of equipment. The size of the force, as measured in active end strength, increased only slightly from 2.0 million to 2.2 million. At the end of the Cold War, the size of the force declined gradually to 1.4 million—the lowest level since the end of World War II—and the budget declined by 35 percent in real terms.

The Post-9/11 budget cycle is fundamentally different because the base budget grew to roughly the same level (adjusting for inflation) as the peak of the Reagan buildup, yet the size of the force did not grow appreciably. The Army and Marine Corps added some 100,000 personnel to deployments to Iraq and Afghanistan in the late-2000s, but these increases were largely offset by cuts in Navy and Air Force personnel levels.Despite the force not growing during the buildup, when the budget began to decline from its FY 2010 peak, end strength began to decline as well. By nearly every measure of force structure—the number of brigades, aircraft, ships and subs, marine battalions, and end strength—the force of FY 2017 is smaller than when the buildup began. Yet this smaller force consumes a budget more than 50 percent larger in real terms than before 9/11. The military is spending more for a smaller force.

Analysis of the FY 2017 Defense Budget
http://defense360.csis.org/analysis-of-the-fy-2017-defense-budget/