As for the fallow up question, answer is quite simple- because Soviet and latter Russian authorities did they best to if not outright deny, then at least to diminish the numbers and existence of GULAG in general. And they are peoples who have all documents, so the rest of the world have to rely on estimations. "Official" number as of now is 2,5 millions at the peak(1950). That is absurdly low number, to say the least. Antonov-Ovsejenko, an extremely anti-soviet author stands on the other point of spectrum with 17 millions. General Volkogonav, I thing, is most trustworthy source (I don't know how much of his works are available in English, thou) and he gives a number of about 4-5 millions and that is similar to R. Conquest's estimation of 5,5 million. Having a problematic access to sources we have to content with this numbers.

A note One problem we face in this matter is what is to counted in to this number. "Sending in to GULAG" in a strict sense refers to placing in forced labor camp complex (like DOLSTROY). But majority of the people were simply forced to relocate (sometimes en masse). Such deportees could not leave the location of the settlement they were assigned to or change work, so for most matters and purposes it was rather a GULAG-light, with main exception that prisoners of GULAG-proper had (mostly) a time sentence, while forced deportation was for life.

Oh and also there was "recruts" from Germany and other countries. Soviets looted German factories taking not only mechanisms, but also forcing German specialists to work in Soviet Union (famous examples is photo cameras of Krasnogorsk Mechanical Works, and off course AK-47).

Bout relocated deportees and foreign specialists were still technically not part of GULAG, but they there taken, deported and supervised by the same NKVD, the reasons (political and economical) for deportation were rather similar if not identical, living and work conditions often were also very similar, so de facto only difference were within bureaucratics of the NKVD. 2.5 million number strongly relies on that by simply excluding this peoples. However many historians- rather reasonably- believes that GULAG-light should be counted too.

P.S Puting all this problems a side peak of the GULAG was post WWII to Stalins death. Next to Axis POW's who were held for years and years, over a million (exact number is, again, long debate) of Soviet POW's were also send to camps after they were returned to Soviets. In 1948 were was another the wave of repression against homeless and displaced war veterans. In addition, there was mass repressions in parts of SSRS that were occupied by Germans during war, going as far as forced deportation of entire ethnic groups (Crymean Tatars and Chechens) for being "fascists sympathetics". Again, much depends of how and what you count, but by all calculations in 1948 GULAG population skyrocketed.