Okay let's just do the fucking math.

The Holocaust took place over five years. We'll lowball it and say 1500 days.

There were about 17 000 000 victims. (6 million Jewish people, 11 million others).

According to Nazi records, there were 1200 camps. I couldn't find numbers for extermination camps but let's say it's 10%, 120 kill camps.

17 000 000 over 1500 days in 120 camps gives you a total of 95 bodies per day, per camp.

A human body produces roughly 5 pounds of ash and bone fragments when cremated, so each extermination camp would produce 475 pounds of remains per day.

That's not much. I mean, you could make a couple of hard workers shovel this onto a truck, drive it out into the woods, and dump it. Or you could turn it into fertiliser by composting it with other waste and a little soil.