Those numbers seem way off, at least in respect to what could be attributed to atheism in any way. Are you suggesting that any and all deaths in an atheist regime can be attributed atheism?

Most of the deaths in China came from stupid policies following the transformation to communism. Most of the deaths during Stalin came from the second world war, in which Soviet was a defender (atleast during the casuality intense second part vs the Axis). Surely famine from transformation to communism and casaulities during defensive wars are equally attributable to atheism as deaths from the black plague nationalistic wars are to religion.

---
Wiki Quotes
---

Not all deaths during the Great Leap were from starvation. Frank Dikötter estimates that at least 2.5 million people were beaten or tortured to death and 1 to 3 million committed suicide.[97]

The true figure of those who were persecuted or died during the Cultural Revolution may never be known, since many deaths went unreported or were actively covered up by the police or local authorities. The state of Chinese demographics at the time was very poor, and the PRC has been hesitant to allow formal research into the period.[72] In their book Mao's Last Revolution (2006), the Sinologists Roderick MacFarquhar and Michael Schoenhals assert that in rural China alone some 36 million people were persecuted, of whom between 750,000 and 1.5 million were killed, with roughly the same number permanently injured.[73] In Mao: The Unknown Story, Jung Chang and Jon Halliday claim that as many as 3 million people died in the violence of the Cultural Revolution.[74] Sociologist Daniel Chirot claims that around 100 million people suffered and at least one million people, and perhaps as many as 20 million, died in the Cultural Revolution.[75]

Accordingly, if famine victims are included, a minimum of around 10 million deaths—6 million from famine and 4 million from other causes—are attributable to the regime,[109] with a number of recent historians suggesting a likely total of around 20 million, citing much higher victim totals from executions, gulags, deportations and other causes.[110] Adding 6–8 million famine victims to Erlikman's estimates above, for example, would yield a total of between 15 and 17 million victims. Researcher Robert Conquest, meanwhile, has revised his original estimate of up to 30 million victims down to 20 million.[111] In his most recent edition of The Great Terror (2007), Conquest states that while exact numbers may never be known with complete certainty, the various terror campaigns launched by the Soviet government claimed no fewer than 15 million lives.[112] Others maintain that their earlier higher victim total estimates are correct.[113][114]

---

It's not impossible that all the conflicts you mention, even with your silly estimates, are shadowed by the largest religious war the last 150 years (by a stretch). While the Taiping Rebellion is usually estimated at 20 million deaths, some estimates go as high as 100 million. These are all deaths due to a war with a big religious component. Whereas removing the atheistic property of the governments in your two prime examples would still mean a large amount of deaths, removing the religious component in this case could well have prevented the war and thereby all its deaths all together.