Why do some people have small heads compared to the rest of their body?

peope have small heads (pea heads) becuase there mum and dad have sex and make them who may also have a small head so two small heads shoul make 1 normal size head but in some sycumstances this may not be true as two people who have ig heads thier child should have extremely big heads but that is not always the case these are called bomb heads.