Macrocephaly is a condition in which circumference of the human head is abnormally large. It may be pathological or harmless, and can be a familial genetic characteristic. People diagnosed with macrocephaly will receive further medical tests to determine whether the syndrome is accompanied by particular disorders. Those with benign or familial macrocephaly are considered to have megalencephaly. Causes
Many people with abnormally large heads or large skulls are healthy, but macrocephaly may be pathological. Pathologic macrocephaly may be due to megalencephaly (enlarged brain), hydrocephalus (abnormally increased cerebrospinal fluid), cranial hyperostosis (bone overgrowth), and other conditions. Pathologic macrocephaly is called "syndromic", when it is associated with any other noteworthy condition, and "nonsyndromic" otherwise.
Below is a list of causes of macrocephaly from Swaimans Pediatric Neurology: Principles and Practice noted in The Little Black Book of Neurology:

Hydrocephalus
Noncommunicating
Arnold–Chiari malformation
Aqueductal stenosis
X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS) syndrome (L1CAM)
Dandy–Walker malformation
Galenic vein aneurysm or malformation
Neoplasms, supratentorial, and infratentorial
Arachnoid cyst, infratentorial
Holoprosencephaly with dorsal interhemispheric sac

Communicating
External or extraventricular obstructive hydrocephalus (dilated subarachnoid space)

Arachnoid cyst, supratentorial
Meningeal fibrosis/obstruction
Postinflammatory
Posthemorrhagic
Neoplastic infiltration

Vascular
Arteriovenous malformation
Intracranial hemorrhage
Dural sinus thrombosis

Choroid plexus papilloma
Neurocutaneous syndromes
Incontinentia pigmenti

Destructive lesions
Hydranencephaly
Porencephaly

Familial, autosomal-dominant, autosomal-recessive, X-linked
Subdural fluid
Hematoma
Hygroma
Empyema

Brain edema (toxic-metabolic)
Intoxication
Lead
Vitamin A
Tetracycline
Endocrine (hypoparathyroidism, hypoadrenocorticism)
Galactosemia
Idiopathic (pseudotumorcerebri)

Thick skull or scalp (hyperostosis)
Familial variation
Anemia
Osteoporosis, severe precocious autosomal-recessive osteoporosis (CLCN7, TCIRG1)
Pycnodysostosis (CTSK)
Craniometaphyseal dysplasia (ANKH)
Craniodiaphyseal dysplasia
Pyle dysplasia
Sclerosteosis (SOST)
Juvenile Paget disease
Idiopathic hyperphosphatasia
Familial osteoectasia
Osteogenesis imperfecta
Rickets
Cleidocranial dysostosis
Hyperostosis corticalis generalisata (van Buchem disease)
Proteus syndrome

Megalencephaly and hemimegalencephaly
Diagnosis
Macrocephaly is customarily diagnosed if head circumference is greater than two standard deviations (SDs) above the mean. Relative macrocephaly occurs if the measure is less than two SDs above the mean, but is disproportionately above that when ethnicity and stature are considered. Diagnosis can be determined in utero or can be determined within 18–24 months after birth in some cases where head circumference tends to stabilize in infants. Diagnosis in infants includes measuring the circumference of the childs head and comparing how significant it falls above the 97.5 percentile of children similar to their demographic. If falling above the 97.5th percentile then the patient will be checked to determine whether there is any intracranial pressure present and whether or not immediate surgery is needed.