How many people was arrested? Oh, 1.5 million according to  the wikipedia. Here is global research for you about the USA. http://www.globalresearch.ca/u-s-prison-population-dwarfs-that-of-other-nations/8801
It has 751 people in prison or jail for every 100,000 in population. So, if population of USA is 320 million and it has 751 in prison or jail for every 100,000 it means that 2.4 million people are sitting nowadays in USA.. Got it? 1.5 vs. 2.4

So why the time when 1.5 million people are arested in USSR called "Great Purge" and the same situation in USA, when the number of people is 2.4 million, called democracy? What is that? Double standards? 

Correctional facilities was invented not by communists. Also why you didn't mentioned trusty system? Or the conditions of prisoners in US was bearable? Don't kidding me.