In 2001 a mass grave from Napoleon’s Russian campaign was discovered in Lithuania, as discussed in the following very interesting paper.
The excavation established that the bodies were buried at a density of 7 bodies per square meter of grave. (Note that that’s square meter, not cubic meter. They are counting in terms of grave surface area.)
According to the paper, the layer of bones is between 20 and 50 cm in depth. A reasonable assumption would be that the average thickness is midway in between those two figures, or 35 cm. Seven bodies per square meter with an average depth of 35 cm means 7/0.35 = 20 bodies per cubic meter. Of course, this is after nearly 200 years of decay! Now, recall that the burial density at Belzec claimed by Roberto Muehlenkamp is more than 20 bodies per cubic meter. But the bodies at Belzec didn’t have the benefit of 190 years of decay to reduce their volume…
Here are some pictures of the bones, which are now arranged with a density of something like 20 bodies per cubic meter.
It should be self evident that contrary to Muehlenkamp’s claims, intact bodies would take much more space than these bones.