What exactly causes loss of skull mass ? i researched and couldn't find it

Depends on finding: Various conditions can result in loss in the skull mass. Systemic disease such as Paget's disease of the bone, bone destruction is much quicker than bone deposition, lead to loss of skull mass. Multiple myeloma, and myeloproliferative conditions can cause punched-out lesions in the skull. Other causes can include: renal osteodystrophy, osteomalacia, amyloidosis, and others, depend on findings.