To western minds, the most common sort of giant we conjure up are derived from Norse and Celtic mythology – more specifically the Norse. The vikings thought a lot about giants, and unlike the Celts, they had the good sense to record their tales prior to becoming Christians.
To the Norse, the giants were the counterparts to the Gods; the loyal opposition. The giants were not necessarily evil (nor were the Aesir and Vanir necessarily good). Rather, the gods the vikings revered fought to create some useful order within the chaos that sprung from the giants.
Daniel McCoy, compiler of Norse Mythology of Smart People explains:
Jotunheim is also known as Utgard (pronounced “OOT-guard;” Old Norse Útgarðr, “Beyond the Fence”), a name which establishes the realm as occupying one extreme end of the traditional Germanic conceptual spectrum between the innangard and the utangard. That which is innangard (“inside the fence”) is orderly, law-abiding, and civilized, while that which is utangard (“beyond the fence”) is chaotic, anarchic, and wild.
The old Norse term for giant, surviving to us as Jotun, roughly translates to “Devourer”, which is chaotic and untidy, but not essentially evil. The Norse saw this as a necessary part of the Cycle of Things: order is carved out of chaos, but chaos inevitably creep back to devour order. Yet from that chaos, order will eventually emerge.
The Norse gods sprang from the corpse of a great giant that had consumed the previous world. So it goes.
Let’s translate that to fur-clad boots on the mythical ground.
The Jotum are a large, proud race of giants dwelling beyond the frigid horizon – in this case Greenland, and the northern wastes of Asia and north America. The center of their lands was the Arctic Ocean.
While relatively uncivilized, they were not stupid, and this, coupled with their large size, could enable them to withstand the arctic conditions of the latter ice ages. But large size brings large appetites, hence their reputation as devourers.
There would not have been a lot of them – needing a lot of territory to sustain themselves. Twenty giants assembled would be an army. But that would be all they would need unless they were assaulting Asgard itself.
Where puny humans see strength in numbers, giants would see only scarcity. They would have little incentive to invent civilization as we understand it. The primary purpose of organized agriculture, after all, is to allow for an increase in population. Also, glaciers will discourage this anyway.
So they hunted and gathered in sparse groups spread across sprawling territories. Even isolated they had nothing to fear but starvation and each other, and the latter would have been an unlikely threat. Every Jotun knew every other Jotun.
One can easily imagine them riding about on the glaciers, atop their mastodon mounts, trained polar bears flushing out seals. Perhaps they herded wooly rhinos the way we herd cattle.
Given that humanoids cap out at eight feet and a thousand pounds or so, a twelve foot, six ton giant could only be sustained supernaturally. Now, let’s go to the Niven theory of magic, that it becomes more prevalent in barren wastes than in settled, or biologically richer lands. Under these assumptions, the giants would need the glaciers just to stay standing.
In the end, the giants overtake the Gods in the Ragnarok, but the Norse saw this as part of a cycle rather than an ultimate end. And the pattern, deep winters, rising oceans, long period of warm calm, receding oceans and deep winters, can be loosely mapped to expanding and contracting ice ages.
The Asgardians might mitigate the Jotun for a while, but they never overcome them. The true enemy of the Jotun is global warming.