What factors make a cranium smaller than avarage head size ? besides genetics i have a small head circumference

Intra-uterine viral: There are several intra-uterine infections causing small head size.These are TOXOPLASMOSIS,CMV,HERPES, and others.

Most are bad: The cranium grows to the size needed for the brain contained therein.The genetic factors that give you a smaller head also produce a normal functioning brain that gets by with less volume.If there is any factor that injures brain development early on, usually the first 12 weeks, like Zika virus/Cytomegalovirus/etc. The brain mass will not grow much and the skull will only grow to reduced size.