Yet another set of mass grave claims can be found in the Soviet report on Katyn:
The Soviets claimed to have found the bodies of 11,000 Poles. They reported that these bodies were in a mass grave of 60 x 60 x 3 meters and a grave of 7 x 6 x 3.5 meters. These figures give a grave area of 3,642 square meters and a grave volume of 10,947 cubic meters. The associated burial densities are 3 bodies per square meter of grave surface and 1 body per cubic meter of grave volume.
While these data cannot be regarded as real, they can be regarded as indicative of the fact that when the Soviets really felt the need to be taken seriously, they made quite modest claims about burial density. This suggests that they realized that the kind of unexampled burial densities claimed at camps like Belzec and Treblinka were not particularly credible.
Aside: As The Black Rabbit of Inlé has pointed out, the sets of authors of the Soviet Katyn report and the Soviet Auschwitz investigation overlapped.