A genocide is not defined by literal death of individuals but for killings to be part of a genocide they have to intentionally target a specific group and its culture.
The Nazi's Eastern front was explicitly a war to create "Lebensraum" (living space) which went with depopulating large areas. They also deliberately initially displaced and then systematically exterminated Jewish populations. They also targeted Sinti and Roma people. For Jews they also went to the lengths of deliberately destroying their cultural artefacts as well as works they claimed to be influenced by Jewish culture.
If you desperately want to pin a mass murder on historical communists, you probably want the Holodomor. But even the Holodomor seems to have at least in part been a case of administrative failure and apparatchiks not taking complaints seriously. But at least there are indicators of some level of malintent even if it may not have been intentionally genocidal.
Unlike the above, the treatment of the Uyghur Muslims does qualify as genocide under most definitions as it actively seeks out to erase culture and traditions even if the settling of Han Chinese people in Uyghur territory may not. But while it involves imprisonment, so-called reeducation and arbitrary arrests, it doesn't involve mass murder.
Note that what the Nazis did to Jews, Roma, Sinti and (to a lesser extent) Slavs wasn't unique in European history either (except for the technology available to them), but Europeans previously only had done this to populations in Africa or the Americas. From the victim's perspective the distinction between fascism and colonialism is at most times completely arbitrary.
But "classicide" is not genocide. I'm not interested in defending authoritarian governments or state capitalist empires. I'm pointing out that it is extremely ill-informed to compare modern China (or even Maoist China) to the Nazis.
> Seven former detainees told the AP that they were force-fed birth control pills or injected with fluids, often with no explanation. Many felt dizzy, tired or ill, and women stopped getting their periods. After being released and leaving China, some went to get medical check-ups and found they were sterile
From what I can gather, the unofficial story behind the mass starvation of the Great Leap Forward seems to be that Chinese bureaucrats fell for exaggerated claims about farming techniques used in the Soviet Union and pivoted the farmers to using those techniques and set quotas based on the expected returns. When the results didn't match the calculations, nobody wanted to (likely, literally) bite the bullet so they tried to make good on the overpromised exports, starving the local populations.
Much like the Chernobyl incident, this was more of a case of bad judgement followed by a rigid chain of command playing chicken with a catastrophe to avoid taking responsibility for a comparably minor gaffe.
Of course the CCP didn't want its bureaucracy to appear incompetent and for the US it was more useful as anti-communist propaganda to frame it as mass murder than administrative failure, so neither side has been particularly honest about it in most "official" material until the end of the (first?) Cold War.
If you want to point out deliberate mass murders by China or the Soviet Union there were plenty of those (e.g. Mao killing the landlords) but in terms of scale they don't really compare.
EDIT: Also if you seriously want to count famines as mass murder, you won't like hearing about what the British did in India.
The official story is false. They knew what was happening -- they had agents in every village, who amazingly were always well fed. There were regular visits and inspections. Look, both the USSR and China had mass famines in the countryside, and it was for the same reason:
They wanted to rapidly industrialize which meant the creation of a large factory worker class in rapidly growing urban areas. This required a smaller and more productive agrarian class with surplus food being shipped to cities.
But rather than wait and let normal urbanization take its course, they simply transferred populations to the city and confiscated all the food that was needed to feed this class, letting a fraction - about 10% of the rural population - starve to death. The remaining farmers were forced to work harder to make up for their dead colleagues and this was accomplished via intimidation and near-slavery conditions. That is, at the end of the day, what forced agricultural collectivization was all about. It was to make up for the fact that a lot of the farmers would not be given food. And everyone understood what this was about.
It was intentional democide to free up mouths to eat in the cities, in order to achieve rapid industrialization.
