When it is hot, smaller is cooler.

In environments where ambient temperature are close to body temperatures, shedding excess heat becomes a big problem for exothermic animals. The linked Scientific American article discusses this in the context of hairlessness.

https://www.scientificamerican.com/article/the-naked-truth/

The largest terrestrial mammals—namely, elephants, rhinoceroses and hippopotamuses—also evolved naked skin because they are at constant risk of overheating. The larger an animal is, the less surface area it has relative to overall body mass and the harder it is for the creature to rid its body of excess heat. (On the flip side, mice and other small animals, which have a high surface-to-volume ratio, often struggle to retain sufficient heat.) During the Pleistocene epoch, which spans the time between two million and 10,000 years ago, the mammoths and other relatives of modern elephants and rhinoceroses were “woolly” because they lived in cold environments, and external insulation helped them conserve body heat and lower their food intake. But all of today's megaherbivores live in sweltering conditions, where a fur coat would be deadly for beasts of such immense proportions.

Human hairlessness is not an evolutionary adaptation to living underground or in the water—the popular embrace of the so-called aquatic ape hypothesis notwithstanding [see box on page 26]. Neither is it the result of large body size. But our bare skin is related to staying cool, as our superior sweating abilities suggest.

Megaherbivores were stuck with their large size for other evolutionary reasons. But in hot Africal, in addition to hairlessness some human types also evolved small body size: the pygmies and the Khoi-San. Consider that heat must largely be shed at the interface between body and environment. As size increases, volume increase faster than surface are and the surface area:volume ratio decreases. It is harder to shed heat.

So too your small humanoids: they evolved in very hot climates and smaller is cooler. The converse is true - if you want to shed heat slowly is is better to be big than small, and larger body sizes are favored.

When it is hot, smaller is cooler.

In environments where ambient temperature are close to body temperatures, shedding excess heat becomes a big problem for exothermic animals. The linked Scientific American article discusses this in the context of hairlessness.

https://www.scientificamerican.com/article/the-naked-truth/

The largest terrestrial mammals—namely, elephants, rhinoceroses and hippopotamuses—also evolved naked skin because they are at constant risk of overheating. The larger an animal is, the less surface area it has relative to overall body mass and the harder it is for the creature to rid its body of excess heat. (On the flip side, mice and other small animals, which have a high surface-to-volume ratio, often struggle to retain sufficient heat.) During the Pleistocene epoch, which spans the time between two million and 10,000 years ago, the mammoths and other relatives of modern elephants and rhinoceroses were “woolly” because they lived in cold environments, and external insulation helped them conserve body heat and lower their food intake. But all of today's megaherbivores live in sweltering conditions, where a fur coat would be deadly for beasts of such immense proportions.

Human hairlessness is not an evolutionary adaptation to living underground or in the water—the popular embrace of the so-called aquatic ape hypothesis notwithstanding [see box on page 26]. Neither is it the result of large body size. But our bare skin is related to staying cool, as our superior sweating abilities suggest.

Megaherbivores were stuck with their large size for other evolutionary reasons. But in hot Africa, in addition to hairlessness some human types also evolved small body size: the pygmies and the Khoi-San. Consider that heat must largely be shed at the interface between body and environment. As size increases, volume increases faster than surface area and the surface area:volume ratio decreases. It is harder to shed heat when you are big.

So too your small humanoids: they evolved in very hot climates and smaller is cooler. The converse is true - if you want to shed heat slowly is is better to be big than small, and larger body sizes are favored.