Why some persons are giant like

Abnormal excess secretion, i.e.hypersecretion of growth hormone, causes gigantism and acromegaly.

When hypersecretion of growth hormone occurs before the fusion of the long bones epiphysis leading to tall stature, hypersecretion of growth hormone causes increases in the size of bones.  Therefore, before puberty, the growth hormone increases the length of bones leading to increased height and is called gigantism.

Acromegaly: hypersecretion of growth hormone after the fusion of the long bones epiphysis, i.e., after puberty, increases bone size, especially bones of hands, feet, and face.  The condition is known as acromegaly.