Giants are an element of folklore, myth and human consciousness easily on par with dragons, albeit with far less exposure in contemporary fantasy. These mighty beings were once widely believed in and greatly feared worldwide.
The origins of giants in legends seem to coincide with the great awe that ancient man had for the natural world. Storms, earthquakes, as well as massive natural wonders, drew from man a need to attach meaning to them. Truly, in ancient times gods and giants were easily interchangeable, being enormous mannish beings of tremendous elemental power.
Many of us grew up hearing of the Greek Titans or Norse Jotun but there were many others. Every culture on the Earth had tales of gigantic beings of human (or human-ish) form that existed in ancient days. Most of these beings helped create either the universe itself, or a given land formation, lake or other landmark.
Most aboriginal cultures attach giant species to natural formations such as the Iroquois Chenoo, shy but violent stone giants, and the alien-like Australian Wandjinas, weather spirits that dwell in the mountains and three times the height of a man.
Ogres in legends are the degenerate descendants of the ancient giants or gods, always posessing tremendous physical strength and sometimes magical powers.
These beings are a far cry from the older elemental giant types.
In some stories, ogres are the survivors of ancient titans, gods, or fallen angels, when all such creatures were defeated, killed imprisoned or otherwise dethroned. Now these remnants are a sickly, savage lot trying to hold on to what power that remains, and bitter about their diminished stature.
Most of the giants found in contemporary fantasy are in-fact Ogres of varying power, being effectively giant, brutal humans who are often horribly stupid. These monsters are almost always ugly and prone to eating intelligent creatures.
Personally, I wonder if the many ogre-ish giants made reference to in ancient stories, particularly of such beings as Goliath, Ajax or any number of supposedly god/demon-descended giants were actually victims of gigantism. Most primitive cultures view any deformity as a sign of a curse, evil parentage, or simple inhumanity. You cant tell me that an ancient ruler at some time somewhere never used a hyper-thyroid giant as a soldier.
So where do these legends come from? In ancient times huge megafauna were hunted and encountered, as were many ape species some of whom are assumed by scientists to have gotten quite large. What do you think?