The reason for this, without getting into the somewhat superficial conversation that people might be drawn to about 'what is the worst?'... is less to do with numbers in the case of Cambodia.

As you say, numbers would seem to indicate a fairly low level comparison between Pol Pot and say, Hitler. Again, I'm going to sidestep the reasons why certain people, or indeed people trying to make certain political points or scoring points on other politically minded people.

What your question boils down to is 'why is Pol Pot in the conversation about the worst things that happened in the 20th century?'

That answer can be fairly convincingly made, in my humble opinion. In fact it was one of the things that first led to my fascination with the period of history known to many as "the Cambodian Genocide" (although I have a few minor issues with using that phrase to describe the majority of things that occurred there). It is the sheer totality of terror that was forced upon the vast vast majority of Cambodians for four years. In many ways it is unique, in many ways it isn't. You mentioned the Great Leap Forward, the Communist Party of Kampuchea often referred to their own project as the Super Great Leap Forward. It was a total upheaval of Cambodian society and included every single person in the country, even those members of the Khmer Rouge themselves were implicated in a complex relationship of victim and perpetrator. It was a more severe version of many of the policies that underpinned the GLF, and was to be undertaken in record time... the reliance on simply killing those who might have been only sent to re-education camps in the PRC or USSR is also somewhat unique. While most current estimates of a death toll in those four years is around 2 million people, with perhaps half of that number being the result of direct violence via the state, that does pale in comparison to say the 30-50 million that died during the GLF.

But not when you consider the population of Cambodia, where this four year period of terror resulted in the death of somewhere around 1 in 5 people, or 1 in 4 depending on the numbers used. Killing around 20 percent of your own population has rarely been attempted by any other regime in the 20th Century, and is one reason why the 'Pol Pot Regime' is remembered in terms of the worst things to happen in that sad century of death.

Another reason why Pol Pot in particular might make that list is the link between a general idea of totalitarianism and mass death. It wasn't war, it was a system of terror and death that was the result of policies decided upon by a very select group of people, of which Pol Pot was the leader. It was every bit as brutal as the other examples you list, but lacking industrialised mechanisms of killing like in Nazi Germany, or a larger variance between a population exposed to the policies like in the USSR or PRC. It was everybody in the country, everyone was implicated, experienced this society. The sheer totality of the regime is remarkable, and when that totality is underpinned by constant back breaking labour, population displacement, erasure of fundamental Cambodian and other ethnic cultural features, not to mention being one tiny mistake away from being arbitrarily killed... that is why it is included.

Again, how much should we pay attention to these kinds of comparisons? Well from an anthropological or genocide studies perspective... well there is a lot to learn. But simple 'Pol Pot worse than x' or 'yeah well x was worse than the Nazis'... yeah that does stray into the territory of not just learning about these horrible periods of the path and trying to make a point by using that suffering for some kind of superficial argument. I'm not saying that you are doing that with this question, but what happened in Cambodia does deserve the gravitas of 'terrible period of mass death' and included in a conversation about these instances of crimes against humanity and genocide in the 20th Century.