What causes the forehead to increase in size other than age and genetic factors?

Bone growth: Anything that causes the frontal bones, alone or secondary to other cranial bone expansion can cause the forehead to grow. Therir or a number of chemical changes too like increased pituitary hormones or increased local growth factors as in neorofibromatosis and other diseases that ause localized overgrowth. You need an evaluation probably including x-rays and perhaps other bone imaging studies. Se.