So, I hope that most horse people are (at the very least) familiar with the 18th century racehorse Eclipse. He was a pretty phenomenal animal who won all 18 of his starts. And this was during a time in which races were often run at a grueling 4 mile distance (in opposition to present day when most horses rarely even run at a mile and a half). He left everyone in the dust nearly every time out.
Now you can argue that because of the distances being run in early racing history horses did not run nearly as fast as nowdays. And because of that it is said that a horse like Eclipse wasn't so phenomenal because his was a test of endurance and stamina, rather than speed and power. But I say, who cares? It's not like stamina over 4 miles in 18 different races is negligent. It's freaking amazing. How many present-day horses could do that now (discounting endurance racers)? Yeah, not so many.
Well, because of his racing record and his breeding record (it is said that over 95% of all living Thoroughbreds today can trace their male line back to Eclipse) he was regarded as a legend. And after his death, his skeleton was mounted in the National Horseracing Museum in Newmarket.
Because we, as humans, feel compelled to find specific scientific reasoning behind every instance of greatness scientists have been studying Eclipse's skeletal structure as well as DNA from his teeth to determine the reason for his prowess on the track. And they have determined (drum roll, please) that he was so dominating simply because of his averageness. Which frankly is totally misleading. If you continue to read the article, it comes out that the scientists built theoretical horse limbs to map out how a horse gallops and determine what factors come into play, structurally speaking, for a horse to safely and successfully run at top speeds. What they learned was that a key ability for a horse to achieve blazing speeds is that the animal can bring its hind legs forward extremely quickly - this seems to keep perfect balance at the gallop and thereby aids the horse in traveling extremely fast. Very tall horses have much more trouble with this. It seems that Eclipse (at approximately 15.3 or 16hh) was just the right size to be a perfect example of this theory.
Now, the scientists appear to be saying that they've concluded that small animals are very fast, but if you ask me Eclipse is not that small. Sure, modern horses might average out larger, but he's freaking 16hh!! It's not like he's some pony. So, I would wager that what the real result is is that Eclipse was correctly proportioned for his size. That his legs were just the right length in proportion to his body so that he could easily maneuver them. And that makes a lot of sense. I can't tell you how many times I've cringed at watching some young, gangly thoroughbred with the longest legs ever gallop around and literally have zero control over the positioning of his legs from one moment to the next. So, yeah, if you put it my way (which is always the right way, mind you) it makes loads of sense. Proportion is everything, especially when a horse is traveling at top speed and the gait inherently means that each leg is off the ground for 80% of each stride. It's going to come down to the most balanced horse. And that horse isn't probably going to be the biggest or the smallest horse, but the one that is the perfect example of what a horse should be.