A short list of nations that committed similar atrocities during the 20th:
Syria, Iraq, China, Vietnam, Cambodia, Guatemala, Japan, Italy, Germany, Algeria, France.
You omitted the genocides committed in North Africa by the Italians. Even so, only China has committed these atrocities on a scale of tens of millions of human beings to equal or surpass those of Russia. Germany, Italy, Cambodia, Uganda, and others were also prolific; but their atrocities and genocides still cannot compare to the scope and horrors of the overall killings by China and Russia. Furthermore, it is Russia and China who continue to perpetrate such crimes against humanity long after the other past perpetrators have reformed and abandoned such crimes.