A hog is a domesticated pig which weighs more than 120 lbs, and it usually refers to a grown up pig. A pig is smaller weighing less than 120 lbs.
In the United States, the term "pig" refers to a younger domesticated swine weighing less than 120 pounds (50 kilograms), and the term "hog" refers to older swine weighing more than 120 lbs. In Great Britain all domesticated swine are referred to as pigs.
Source: Foods and Nutrition Encyclopedia, 2nd ed , vol. 1, p. 1184.