OK so unarmored planetoidesimal suffer a 20% loss due to the hull material, armored devote 50% of the hull volume. The hull is cheap, the mass and volume penalty imposes costs once you start making it mobile. There are general classes of these objects nickle-iron fully differientated, in other words a 100% Ni-Fe alloy body, next is the partially differientated Ni-Fe mixed with lithic materials, all molten at some point and then cooled and cryatalized slowly. Next up is the fully lithic materials, all rock in one solid chunk. Then we have lithic fragments embedded in a matrix of ices, and finally a solid chunk of various ices.