A 1/16-scale block of rock (or tank) is 1/16th its length, 1/16th its width and 1/16th its height. Dividing the rock's weight by 16 simply gives you the weight of a cross-section (e.g. a slice which is 1/16th the length but still at scale width and height). Dividing by 16x16x16 is correct.

Gerry