So you are willing to grade muss murder by numbers but you are not willing to compare methods of execution. That is my point: it is the killing that is grossly immoral, not the method.
You are wrong. They are both evil, and some more evil than others.
This is not a contest. Victim count is the only way I'm prepared to make direct comparisons between the Holocaust and the mass murder by Communist states. The Communists killed far more.
But you said something interesting before, about whether Williamson denied the moral dimension of the Holocaust--singular. I think that illuminates my position. There are many moral dimensions to any evil act, all the more true when speaking of the evil programs of a state.
There are aspects of the vernichtungslager that was not present in the Soviet state--systematic industrialized annihilation for annihilation's sake, as an act of genocide. The only thing Soviet that comes close is the imposed famine in the Ukraine (Holodomor).
Arguing about which is more evil is akin to disputing the number of angels on a pin. What is important is to recognize the manner--all of the manners--in which each program is evil.