What environmental conditions would cause large humans to evolve?

Question

For most of our evolutionary history, we did indeed live in Africa - but not just the eastern savannahs, as previously thought: our biological ancestors were distributed everywhere from Morocco to the Cape. Some of these populations remained isolated from each other for tens or even hundreds of thousands of years, cut off from their nearest relatives by deserts and rainforests. Strong regional traits developed. The result probably would have struck a modern observer as something more akin to a world inhabited by hobbits, giants and elves than anything we have direct experience of today.

"The Dawn of Everything", David Graeber and David Wengrow

What environments might cause a larger variety of human, on the scale of 8 to 10, or possibly even 12 feet tall, to evolve?

I presume that humans haven't evolved to average heights of more than about 6 feet because A) humans can adapt to different climatic conditions by developing new tools and strategies, rather than through physical adaptations like growing very large or very hairy, or B) simply because enough time hasn't elapsed for Homo sapiens to evolve that way. That being said, I know that some populations have adaptations for high-altitude life, and that Inuit have genetic mutations which make them more adapted to cold and a high-fat diet, which same mutations also affect height. Nevertheless, aside from some outliers, humans on average don't get much bigger than about 6 feet. How could sizes beyond this evolve?

Could very large, stocky humans evolve in a cold climate, yeti-style? Or would adaptation to a cold climate be more likely to have the opposite effect?
Or, would large size be more likely to evolve in hotter climates? Could a population of humans populating a hot savannah evolve to 8-10 foot height, in a similar way to how African megafauna evolved great size? Perhaps humans could attain great sizes in hot, humid environments where biodiversity is high and food is plentiful? I believe I read that average human height has increased in the past century because of better nutrition in the modernized world--but for the sake of this question, let's assume our hypothetical megahumans don't live in this kind of world--rather, these populations of larger humans may spend more time on the move, settling in one place for a few months out of the year at most.

Answer 1

Island gigantism

In Earth's natural history, primates are moderate length

In the comments, I've pointed out body length has never been larger than 10 Feet, for Earth's primates. I think this is caused by the ape's basic body plan, which allows for speed and agility. The ape became a strong armed creature able to carry its own weight, dwelling in trees.

Environment

As the opener pointed out, body length can develop when evolution takes place. When large, bipedal humans can make good use of an environment that allows for 12 Feet length. This will rule out dwellings like dense forests, trees, and most caves, but open grass land would do fine. It will deter most predators. Some large predators would pose a problem, though.. 12 Feet will render your bipedal primates visible for e.g. packs of lions, from large distances!

A disease: Gigantism

As a result of the excessive amounts of growth hormone, children achieve >heights that are well above normal ranges.[12] The specific age of onset for >gigantism varies between patients and gender, but the common age that >excessive growth symptoms start to appear has been found to be around 13 years

https://en.wikipedia.org/wiki/Gigantism

EDIT: this is pathology, not the evolutionary advantage you seek. Gigantism has side effects, some parts of the body like bones and heart may lag behind, resulting in weakness and fatigue. The king of Prussia found out, he assembled the Potsdam Giants, a regiment of taller than average soldiers.. not very successful,

Although Prussia briefly intervened in the Great Northern War, the Potsdam Giants never saw battle during his reign. Some sources state that there was a military reason to create a regiment of "long fellows" because loading a muzzleloader is easier to handle for a taller soldier. Another source states that many of the men were unfit for combat due to their gigantism.

A special type: Island gigantism

In some rare cases, a species will migrate and find no enemies in its new environment. It emigrated to an island, or isolated region, starting out with a small population, with little competition for food. When predators are too small, or absent and food is abundant, ever larger individuals will survive. An early primate example is the giant lemur Archaeoindris which was as large as a gorilla (6 Feet), while the other lemurs measure 1-2 Feet.

https://en.wikipedia.org/wiki/Island_gigantism

Humans, humanoids

The gigantism disease exists in humans, there are examples listed in the Guiness Book of Records. The excessive length will hinder humans: they have settlements designed for a smaller size, and the larger body will make communication with other humans difficult.. and the larger body will require more food. Humans suffering of gigantism disease will die soon, in early pre-agriculture societies.

Now suppose it's much earlier in human history.. hunter-gatherers, a group of humans has found an island in the early stone age, say Oldowan culture 1-2 million years ago. In a certain period, climate allowed them to migrate to the island, which has plenty of food and no bears or tigers. A similar isolated development could occur in cold, mountainous regions, like the Himalaya. This is makes Yeti a plausible myth! As other answers already mentioned, there's a scenario where Yeti got killed by other, smaller humans. An isolated environment also means they'll be smaller numbers.

It can go in either direction

I have to note Yeti was never found, but the opposite of gigantism, which is insular dwarfism, seems to have existed in early humans. Opposite circumstances: they dwell in caves, food would be sparse and small size would be benevolent, for instance because certain predators would become less interested to prey upon little humans. This may have happened to the Flores people.

Answer 2

1. It has been found by some archeologists that a rise in oxygen level has been a factor in the ability for species to grow bigger.

2. Animals have cells that generate heat. Heat goes out from skin to air contact The bigger the animal, the lower the skin to mass ratio is. So it's harder for bigger animals to get rid of the heat their cells create. Being in a colder environment would be beneficial to taller human.

Bigger animals have slower cell metabolism, to help reduce the heat generated, which helps with the average lifespan. Maybe the bigger human would live longer, reproduce slower.

3. Lower gravity would certainly help since there is a limit to the resistance of our body to its pull. It also costs extra energy to move.