A 'midget' is small (but in proportion) because of a deficiency of pituitary growth hormone - a condition generically known as hypopituitarism. A person with an abundance of growth hormone would be a giant.

A 'dwarf' is short and not in proportion because the condition ( achondroplasia ) is caused by a lack of growth of the long bones of the arms and legs - the trunk is normal sized.

These (and many more) definitions from the Little People of America web-site.

Achondroplasia Of the estimated 200 types of dwarfism, achondroplasia is by far the most common, accounting for approximately half of all cases of profound short stature. Achondroplastic dwarfism is characterized by an average-size trunk, short arms and legs, and a slightly enlarged head and prominent forehead. Most achondroplastic dwarfs are born to average-size parents, and account for somewhere between one in 26,000 and one in 40,000 births. Adults, on average, are four feet tall. Young children, especially, should be examined for such potential problems as central apnea, obstructive apnea, and hydrocephalus.

Growth-hormone deficiency Also known as hypopituitary dwarfism, hypopituitarism, pituitary dwarfism, and panhypopituitarism. Children with growth-hormone deficiency often grow normally until they are two or three years old, then fall behind their peers. Growth-hormone deficiency affects an estimated 10,000 to 15,000 people in the United States. Difficult to diagnose, it can be treated with regular injections of human growth hormone. Unlike skeletal dysplasias such as achondroplasia, diastrophic dysplasia, and SED, growth-hormone deficiency causes a short-stature condition in which a person's head, trunk, and limbs are in the same proportion as an average-size person's. A person with this appearance used to be known as a midget, although that term is now considered offensive.