In about 1965, the United States stopped increasing the number of its deployed nuclear weapons, having reached just over 30,000 of them. Every year since then, the U.S. has reduced its stockpile of deployed nuclear weapons, or held it constant, so that today it has fallen by 2/3 from its high-water mark and the U.S. maintains a strikeforce of just over 10,000.
The USSR, however, continued adding to its stockpile for twenty years after the U.S. stopped doing so, and indeed halted the expansion only when it went bankrupt and collapsed. By the late 1980s, when the U.S. arsenal held just over 20,000 weapons, the Russian arsenal was twice as large, over 40,000. It plummeted precipitously after the USSR collapsed, and today the U.S. force is slightly larger than Russia’s. More than half of all the nuclear weapons that exist in the world today are either active or inactive Russian weapons.
Source: Time magazine, October 23 issue, pages 34-35, relying on the Carnegie Endowment for International Peace and the Natural Resources Defense Council.