Jumjumah denotes the skull as a bony structure. ʿAlāh is a heavy solid stone, an appropriate model of firmness. Mibrad means a file, an implement used to abrade and shape hard material.
The three words establish one image of resistant substance and closely shaped edges. Consequently, “the head is large” is too thin a translation. The important features are strength, orderly construction, and the meeting of parts. The imagery should be understood as a figurative description of anatomy.