One correction, unrelated to gardening...
This is not accurate:
What you receive back after cremation
is a small amount of what is left from
a body burning
You absolutely receive all of the ashes. While a dusting might remain after sweeping out the retort, imagine how people would feel about workers throwing away the bulk of their loved ones ashes. Oh boy.
Most adult cremains will weigh weigh about 4 to 8 lb. You can estimate the volume by the person's weight, typically 150 lb person's ashes will be around 150 cubic inches.
Depending on the crematory's cremulator and process, much of it may be fine powder, but typically it is more granular and may even have small chunks of bone.