A generous overestimate of the volume of ashes from a cremated adult is 4 liters (most sites quote between 3-3.5 L). So 900K would require 3.6 million liters. A cubic meter is 1000 liters, so that is 3600 cubic meters. There are many ways to slice that, for example 60 m wide x 60 m long x 1 m deep. Not saying the meme is wrong, just that its line of argument isn't super convincing.