degradation of Greek cavalry in Hellenistic era

[abou]

While I am not an expert on horses, there is a general trend in the domestication of large animals that the first domesticated types are actually smaller than the wild types, then over time the size increases. This is easy to understand if you consider that smaller types are more manageable, and early in the development of animal husbandry this is a big plus.

It isn't simply that they are more manageable. All the evolutionary precursors of horses have been quite small; utterly diminutive for the earliest. You work with what you can get, and clearly it took a long time of selective breeding to get larger horses. This was likely helped with the feeding of concentrates -- namely barley -- to offset the nutritional needs of these larger, working horses.

Also, the local varieties of animal in places where they were first domesticated may have been small due to local conditions. For example, wolves at lower latitudes are generally smaller than northern wolves. Modern cattle are still generally smaller than the Aurochs of europe (now extinct).

It isn't so much a north v. south comparison, but really the temperatures of the climates. The reason for larger and smaller body sizes is thermoregulation. A larger, heavier animal will have less surface area per unit weight than a smaller animal. This means less radiation of heat into the environment, which is a benefit in cooler climates and vise-versa in warmer climates. Therefore, since less energy is consumed for thermoregulation, more can be used for other activities.