The US's loss of nuclear superiority, at least in terms of raw numbers, was the result of several different factors ranging from changes in defense doctrines, fiscal retrenchment, and the geopolitical situation.

During the 1960s, US Secretary of Defense McNamara led a pushback against the Eisenhower era's policy of massive retaliation. McNamara felt that the policy of massive retaliation was much too limiting and placed far too high a priority upon a massive first strike as a deterrent. Instead, McNamara's misnamed "no-cities" strategy prioritized the ability of the US to possess the capability for a second strike. This necessitated the further development of the nuclear triad, in particular SLMBs, as a means to assure that US nuclear forces would survive a Soviet first strike. The structure of US nuclear forces was no longer predicated upon maintaining the numbers suitable for a massive first strike, but rather a nuclear defense in depth. The Soviet build-up in nuclear forces by the 1970s led to a revision of US nuclear doctrine, NSDM-242, sometimes called the Schlesinger Doctrine. NSDM-242 further emphasized the ability to use nuclear weapons in a flexible manner. One aspect of NSDM-242 was its emphasis upon using nuclear weapons in a targeted manner. The American nuclear doctrine of the 1960s emphasized that the US would not strike first in a nuclear exchange, but this was a doctrine born from American nuclear superiority. Brezhnev's race to nuclear parity undercut this confidence and a component of NSDM-242 was developing the capability for NATO to use nuclear weapons first if it became apparent that conventional forces in Europe had failed. NSDM-242 also called for attacking Soviet industrial targets and later evolved into an emphasis upon attacks on Soviet leadership under the Carter administration's revision of US nuclear strategy, PD-59.

The looming spectre behind this evolution in US nuclear policy was the Vietnam War. One of the novel features behind NSDM-242 was that its emphasis upon multiple platforms gave the American president the option of using nuclear deterrent in peripheral regions like the Middle East or East Asia. Yet, the main impact of Vietnam on American nuclear strategy was fiscal and political. The escalating costs of the war and increasing congressional oversight over the defense budget meant investing in new nuclear systems or maintaining American numerical superiority was quite difficult. Both McNamara and Schlesinger's nuclear doctrines reflected this fiscal retrenchment as they stressed doing more with less. The advent of MIRVs and the success of the Polaris/Poseidon SLBM program imparted some flexibility to the US nuclear doctrine, but it had become increasingly clear that US nuclear forces were becoming long in the tooth by the Carter administration. The new investments in nuclear forces in the late 1970s such as the B-1 bomber or MX missile became intense hot-button political issues. Although these programs were eventually funded, they underscored to American policymakers that they could not take political support for nuclear expansion for granted.

For their part, the Soviets faced a different dilemma when evaluating the nuclear threat. The men of Brezhnev's generation had come of age during Stalinism and the Second World War and these both proved a formative experience for this cadre of Soviet leaders. The surprise attack of Barbarossa cemented in Soviet strategic thinking the importance of a defense in depth and contingencies for the worst case scenario. From Moscow's perspective, parity was not just a matter of matching the United States, but also Western Europe, the People's Republic of China, and other potential nuclear threats such as Japan. The success of the Polaris program deeply alarmed the Soviets as they not only had to contend with American submarines, but the SSBNs of Britain and France's indigenous M1/M2. This was part of the reason why the Soviet conventional naval build-up concentrated heavily upon ASW capabilities. The development of the Tomahawk missile and other cruise missiles deeply alarmed the Soviets as it added a further dimension to a potential nuclear threat. The small size of these cruise missiles and their apparent accuracy meant that in addition to SSBNs, NATO could mount nuclear delivery systems on smaller platforms like SSNs or surface ships. The Soviet response to these threats was to both expand both conventional forces and develop its own version of the nuclear triad. Although the Soviets would continually rely upon land-based systems, the Soviets increased funding on SSBNs and to a lesser extent, bombers during the 1970s.

This Soviet build-up in both conventional and nuclear forces did not go unnoticed in American politics. Although American policymakers had largely given up on nuclear superiority, American politicians were far from content to countenance ceding nuclear superiority to the Soviets. The message of this 1980 Reagan campaign ad was that "peace is made by the fact of strength-economic, military, and strategic." Reagan used Carter's cancellation of the B-1, done because of advancements in ALCMs and low observable technology, as further evidence of America's military weakness in his campaign. Publications like the Department of Defense's Soviet Military Power repeatedly harped upon the idea that the Soviet's were on the verge of a crushing nuclear superiority. This nuclear alarmism seeped into American popular culture as well within the emerging genre of the technothriller. Bestsellers like The Hunt for Red October and Flight of the Old Dog prominently featured Soviet superweapons that could translate into a Soviet strategic victory if the balloon went up. Although Reagan's military build-up remained quite contentious in American politics throughout the 1980s, the number of Soviet nuclear weapons was not taken lightly by American policymaker.

Sources

Freedman, Lawrence. The Evolution of Nuclear Strategy. New York: St. Martin's Press, 1981.

Sagan, Scott Douglas. Moving Targets: Nuclear Strategy and National Security. Princeton, N.J.: Princeton University Press, 1989.

Terriff, Terry. The Nixon Administration and the Making of U.S. Nuclear Strategy. Ithaca: Cornell University Press, 1995.

Zaloga, Steve. The Kremlin's Nuclear Sword: The Rise and Fall of Russia's Strategic Nuclear Forces, 1945-2000. Washington: Smithsonian Institution Press, 2002.

The Dead Hand is a fantastic book. The answer to the first part of your question is pretty simple though. The U.S. wasn't content with this land-based imbalance but it also wasn't concerned about it. The U.S. and Soviet nuclear force compositions were organized differently. The Soviets relied overwhelmingly on land-based launchers and missiles. The U.S., however, adopted a triad strategy of land, sea, and air based launchers and missiles. The idea being that the Soviets were less likely to effectively counter or wipe-out a triad system as opposed to a system focused on one type.

In terms of overall nuclear force, the U.S. and NATO always had more while the Soviets typically had more land-based missiles. The Soviet Union simply did not have a navy and air force capable of out-doing the West. The differences in these force compositions were what made the arms limitation and arms reduction negotiations so difficult. A hard cut in one type of nuclear forces dis-proportionally affected one side.

I can't answer about the destructive power of each side's forces. Hopefully someone else will. I hope my answer gives you a general idea of how these forces were structured though, helping the discussion along.

Strobe Talbott's Deadly Gambits, in addition to The Dead Hand and Don Oberdorfer's From the Cold War to a New Era offer some pretty good discussions of arms control issues.