Do horses shrink in the dryer?

A post-apocalyptic future caused by climate change is a popular Dystopian theme. Perhaps these future animals should be smaller, at least in areas of increased temperature. Previous periods of global temperature increases have reduced the size of horses up to 30%, according to this brief article from Science. When the global trend reversed, animal size rebounded.

Of course evolutionary pressure favoring smaller animals takes place over long periods of time. But in the short run, maybe we would see evolutionary pressure killing off a great number of larger animals.

Larger muscle mass could mean more heat production, but would the animals die of heat stroke? Or would there be a more complex mechanism. Look at what is happening to Moose, the largest deer species. Their population is declining precipitously due to two parasites and white-tailed deer, but the parasites have always been around, they're just more abundant now due to climate change.

For domesticated animals, who can benefit from modern medicine and human-controlled breeding, will these ecological factors be mitigated? If so, horses and cattle might not increase.

A picture is emerging of our microbiome (the symbiotic bacteria that live in our guts) that shows the impact of our diet on our health. We feed domestic farm animals on diets that maximize their growth, so farmers can make more money on each animal. Then farmers and veterinarians administer antibiotics to help them avoid illness and grow larger. But perhaps it's healthier for an animal to be smaller, and perhaps livestock should have a diet that maximizes animal health rather than size. We're already counteracting the environmental pressures on animal size.