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.
If excess fluid is found between the ventricle spaces in the brain then surgery will be needed. Associated syndromes
Below is a list of syndromes associated with macrocephaly that are noted in Signs and Symptoms of Genetic Conditions: A Handbook. Include multiple major and or minor anomalies
Acrocallosal syndrome
Apert syndrome
Bannayan–Riley–Ruvalcaba syndrome
Cardiofaciocutaneous syndrome
Chromosome 14 - maternal disomy
Chromosome 22qter deletion
Cleidocranial dysostosis
Costello syndrome
Encephalocraniocutaneous lipomatosis
FG syndrome
Hallermann–Streiff syndrome
Hydrolethalus syndrome
Hypomelanosis syndrome
Hypomelanosis of Ito
Kelvin Peter anomaly plus syndrome
Lujan–Fryns syndrome
Macrocephaly-CM (MCAP)
Marshall–Smith syndrome
Neuhauser megalocornea/MR syndrome
Neurofibromatosis type I
Nevoid basal-cell carcinoma syndrome
Noonan syndrome
Ocular-ectodermal syndrome
Osteopathia striata - cranial sclerosis
Perlman syndrome
Robinow syndrome
Simpson–Golabi–Behmel syndrome
Sotos syndrome
Sturge–Weber syndrome
Weaver syndrome
Wiedemann–Rautenstrauch syndrome
3C syndrome

Secondary to a metabolic disorder
Glutaric aciduria type II
GM1 gangliosidosis
Hunter syndrome
Hurler syndrome
MPS VII
Sanfilippo syndrome
Zellweger syndrome

Associated with a skeletal dysplasia
Achondroplasia
Campomelic dysplasia
Craniodiaphyseal dysplasia
Craniometaphyseal dysplasia
Hypochondrogenesis
Hypochondroplasia
Kenny-Caffey syndrome
Kniest dysplasia
Lenz–Majewski syndrome
Osteogenesis imperfecta III
Osteopetrosis, autosomal recessive form
Schneckenbecken dysplasia
Sclerosteosis
Short rib syndrome, beemer-langer type
Short rib-polydactyly 2 (majewski type)
Spondyloepiphyseal dysplasia congenita
Thanatophoric dysplasia

With no obvious physical findings
Alexander disease
Canavan disease
Cobalamin deficiency (combined methylmalonic aciduria and homocystinuria)
Dandy–Walker malformation
Glutaric aciduria type 1
L-2-hydroxyglutaric aciduria
Megalencephalic leukoencephalopathy with subcortical cysts
Osteogenesis imperfecta IV
Osteopathia striata-cranial sclerosis
Periventricular heterotopia
Sandhoff disease
Tay–Sachs disease

See also
Microcephaly
Megalencephaly
Hydrocephalus

References
External links
GeneReviews/NCBI/NIH/UW entry on PTEN Hamartoma Tumor Syndrome (PHTS)
GeneReviews/NCBI/NIH/UW entry on 9q22.3 Microdeletion