It is not so much a matter of less material that exists (although no doubt the CIA was the more bureaucratic paper factory of the two), but that the CIA is quite a unique intelligence service for regularly reviewing its own classified materials and publishing documents that are no longer deemed sensitive. Sometimes US oversight institutions and courts have also aided this process of declassification.

Despite that, the most sensitive operational materials of the CIA remain classified, and in the history of intelligence, one must always bear in mind that the most sensitive operations leave the smallest paper trail. What one finds in the majority of declassified documents are finished reports, memo's, estimates, intelligence products that have been sanitized and contain no sensitive operational secrets so that they can be safely circulated among the intelligence community or supplied to policy-makers and politicians. Believe it or not, the CIA is in fact one of the most open and transparent Western intelligence agencies. The UK has released much fewer materials, and until 1988 even formally denied the existence of its intelligence agencies.

Germany has rigorous oversight, but remains secretive, and in France the intelligence community also remains shrouded in mystery, despite being occassionally rocked by (corruption) scandals.

The KGB in the Soviet Union was of course accountable to the Communist Party, until the latter decided to abbandon the entire project while the former had just succeeded in penetrating the "Adversary" deeper than ever. So much for being the "Sword and Shield" of the Revolution. The KGB's publically available files basically mostly consist of leaks. The first serious batch of leaked material now available in book form came from Oleg Gordievsky, the KGB officer who toured Europe since the 1970's and was recruited by MI6, until he eventually had to defect in 1985 because he (probably) got betrayed by one of those KGB moles inside the CIA's Soviet division.

Other defectors have brought with them files and documents too, it is their bargaining chip after all, but most of those remain behind lock and key.

After 1991, as the Soviet Union collapsed, more leaks occurred. One of them is now known as the Bukovsky Archive. It contains the stolen copies from the archive of the General Department of the CC. Let me clarify: the KGB was a spy organization in the purest sense of the word - it recruited and handled spy networks. Unlike the CIA, its analytical capacity was limited - a few hundred analysts at most. The CIA had half of its organization consist of analysts. The overwhelming amount of KGB reports was therefore basically raw intelligence. Its analysis and dissemination occurred within the administrative apparatus of the Communist Party. First the Information Committee, later the International Department and the General Department (comparable to the Presidential White House Staff in America) was responsible for archiving top secret materials.

The General Department supplied Politburo members for example before a meeting with secret KGB documents, and expected the copies back within a few days. So much of the KGB's archive was in fact located in the archives of the Central Committee. Some of those ended up in the West, and are online available, mostly in Russian and poor copying quality.

During those same years, a frustrated KGB archivist named Vasili Mitrokhin was responsible for the largest intelligence leak in history. As the KGB moved to its new building at Yasenovo in the 1970's, Mitrokhin decided to make use of it by keeping notes of all the archived materials he transferred. The notes he kept hidden in places where the sun doesn't shine, and when in the 1990's chaos reigned, he travelled to the West, made contact with MI6 to help him smuggle his papers out of the country. In collaboration with historian Christopher Andrew a book was written on the basis of these materials and appeared in 1999, and since then some transcripts of Mitrokhin's notes have been published. The majority of the so-called Mitrokhin archive remains a secret treasure of MI6.

During the early 1990's, Russia also opened its archives to historians. From this episode we have the so-called Vassiliev Notebooks, a pile of notebooks containing notes made by Alexander Vassiliev, mostly of documents relating to Soviet espionage in America until around 1960. Much of what we know of Soviet espionage in the 1930's and 40's is based on these materials.

By the mid 1990's however, the KGB and Communist Party archives were closed off again as the post-Cold War honeymoon was over and the Russians realized the West had no intentions in reciprocating the disclosures of Cold War secrets. The SVR, the KGB's succeeding agency, also began to release its own multi-volume histories and occassional disclosure of documents.

On top of that, some KGB files have indirectly been disclosed through the archives in Eastern Europe. Since the intelligence agencies of East Germany, Czechoslovakia, Bulgaria and Hungary were basically extensions of the KGB, their archives also contain instructions and plans, priorities and shared reports from the KGB. Especially East Germany's archive has disclosed as much as possible, although its leaders ordered the destruction of its foreign intelligence files. Some of it has been preserved, and we now have for example copies of Operation RYAN Reports from the KGB shared with the East-German MfS.

Lastly, the US has also been publishing the intercepted Soviet diplomatic cables from the VENONA program between 1943 and 1948, through which we can now enjoy the chatter between Moscow and its various stations in the US and Europe, reporting for instance on an agent inside the British embassy in Washington referred to as "G", of whom we now know it was Donald Maclean from the Cambridge Five spy ring.

So to conclude, the CIA is simply disclosing much more material on much more regular basis. At the same time, its not really their most sensitive material. The KGB's files nowadays are mostly sealed off to the public, but what has come out, came largely through leaks and in fact contained precious secrets and details of operations.

As far as Signals intelligence is concerned (materials from the NSA and the KGB and GRU's signals units), virtually nothing has been declassified or disclosed. Partly because it is too sensitive as it would threaten ongoing methods of interception, but also because of this sensitivity, many materials have been destroyed immediately after use.