The Hungarian Revolution was a "harder" revolution, in the sense of overthrowing the government and desiring to leave the Soviet sphere of influence completely. Yet about 30,000 Soviet troops and KGB agents were able to suppress the revolt, inflicting massive casualties (over 3,000).
By comparison, the 1968 invasion of Czechoslovakia was a Bloc-wide affair, with up to 500,000 total troops (250,000 of which came from the USSR), despite it being a "softer" situation: Czechoslovakia only wanted internal reforms rather than to fully depart the Bloc, revolutionaries didn't overthrow the government, Czechoslovakian troops didn't formally engage the invaders, and about 150 people died - 5% as few as the amount of casualties in Hungary in 1956.
Why did the Soviet Union manage to crush Hungary with so much fewer troops; or, conversely, why did they use such an overwhelming force on comparatively tiny Czechoslovakia in 1968? Was it a show of force, either to other Warsaw Pact members (unity against dissent) or to the Western powers against the backdrop of the Cold War and a potential conflict in Western Europe? Was it an attempt to use overwhelming force projection to stop a more active Hungary-style revolution before it started? Is the higher troop number causally linked with a lower number of casualties compared to Hungary due to the above deterrent effect?
The harsher response to the Prague Spring was precisely because of the violence of the Hungarian Revolution, despite the fact that that revolution was ultimately suppressed. 1956 had shown the Soviet leadership that the communist regimes of the Eastern Bloc were still vulnerable to revolution and that those revolutions could turn violent and have consequences for the communists even if they failed. Even though the reform movement in Czechoslovakia wasn't turning violent, Brezhnev's reflexive response was to stamp it out to avoid another Hungary.
Some historians refer to a "Hungarian Syndrome" or "Hungarian Complex" among Soviet leaders, particularly Yuri Andropov (later head of the KGB and General Secretary of the CPSU), who was the Soviet ambassador to Hungary in 1956 and saw Hungarian secret policemen (AVH) hanged from lampposts by the revolutionaries. Andropov played a leading role in convincing Khrushchev to intervene in Hungary, and was subsequently one of the leading forces behind the scenes who pushed Brezhnev to move decisively to suppress the nascent revolution in Czechoslovakia, even though it was more peaceful than the Hungarian Revolution had been. Andropov was also responsible for the political repression that followed the Prague Spring (known as the Normalization).
The Soviet leaders were also afraid that reform movements in Eastern Europe could weaken those communist regimes and move them away from Soviet domination, which would in turn (in the Soviets' estimation) weaken the bulwark that the Eastern Bloc provided against the West. While the Soviets were initially willing to negotiate with the new Czechoslovak government, with the understanding that they were still committed to communism, the hardliners within the regime (particularly Andropov) pushed for a more aggressive response, which Brezhnev ultimately supported.
The Soviet and supporting Warsaw Pact forces (from Poland, Hungary, and Bulgaria; Romania and Albania refused to participate) had a significant numerical advantage, as well as some element of surprise (since the Czechoslovak leadership believed the negotiations had successfully convinced the Soviets not to invade and made no defensive preparations), ultimately the main reason for their success was the fact that the Czechoslovak leadership decided not to respond in kind and resist the invasion. The memory of the Hungarian Revolution also affected their decision-making, because they wanted to avoid the bloodshed that had occurred in Hungary.
The harsh Soviet reaction to the Prague Spring was the beginning of the Brezhnev Doctrine, which supported the use of force to maintain communist rule in the countries within the Soviet sphere of influence. The most notable example aside from Czechoslovakia was the Soviet invasion of Afghanistan in 1979, which led to the disastrous ten-year war there. However, the Soviets didn't respond with force to the Polish political crisis that stemmed from the emergence of the Solidarity trade union in 1980, which more or less marked the end of the Brezhnev Doctrine, even though the war in Afghanistan continued for the rest of the decade, well after Brezhnev's death in 1982.
Sources:
Gunter Bischof, et al., The Prague Spring and the Warsaw Pact Invasion of Czechoslovakia (Lexington, 2010)
Paul Lendvai, One Day that Shook the Communist World: The 1956 Hungarian Uprising and its Legacy (Princeton UP, 2008)
Matthew Ouimet, The Rise and Fall of the Brezhnev Doctrine in Soviet Foreign Policy (UNC, 2003)