While this question could come as a surprise for the non-initiated, anyone who has spent time on or around horses will know horses are forever getting injured, lame and sadly often worse.
Horses are mostly big animals, seen as powerful and fast. Why is it, then, that they are so fragile? The answer is actually in the question: it's precisely because horses are big and fast that they are so fragile.
Millions of years ago, horses used to live in forests. They were much smaller and ate mainly vegetables and fruits. Gradually, their habitat changed to grass land. To survive, horses became bigger as it required larger bodies to process the hard-to-digest cellulose in the grass. They also required longer legs to be able to run away from predators over longer distances.
What we think the horse looked like 50 million years ago
Over time, and more recently through human’s selective breeding, horses developed to be extremely good and fast runners. Mechanically, thinner legs are more efficient which means that, relative to their size, horses have thin legs and therefore fragile ones too. If you combine this with the fact that they are extremely flighty (their first instinct is to run away) then you have a perfect recipe for broken legs.
The horse's body in motion at a canter - built for speed
Secretariat - one of the fastest race horses
It is very complicated and costly to fix a horse's broken leg. First because of how fragile it was in the first place but also because horses are not able to rest, they need to be constantly be on the move as their feet act like a pump which distributes blood throughout the body. If a horse can’t move its survival is at risk, which makes fixing and healing an injured leg problematic. And even if you did fix it, chances are the horse will never be able to run as it used to. This is why many owners chose to put their horses down, for both economic and humane (avoid long lasting pain) reasons.
The second reason horses are fragile is because of their digestive system. Here, again, the answer can be found in evolution. As mentioned earlier, their bodies had to adapt to eat grass which is hard to digest and poor in energy. Cows, sheep or deer also eat grass but they are ruminants which means they digest grass through fermentation in their multi-chambered stomachs. Horses, on the other hand, are what we call “hind gut fermenters” which means that they only have one stomach (like us) but have developed a system in which they can digest cellulose in the hind gut.
It’s precisely because they don’t have the heavy digestive structure of cows or other ruminants that horses can run like they do. But this comes at a cost: they have to eat lots of smaller meals (namely grazing) which will trickle through their digestive tract throughout the day. Horses can’t lie down for an extended amount of time as it disrupts this digestive system. A change in diet, or suddenly eating a big quantity of food as well as drinking too much water at the wrong time will disrupt the digestion.
Unfortunately, a digestive problem in horses often escalates. Horses can’t vomit, they are physically incapable of doing so. Some say it’s another evolutionary tweak to allow them to run from predators - otherwise the back-and-forth motion of their body when they run could make them vomit. Since they can’t vomit, and since their digestive system works like a trickle system, any food impaction (due to something not digested well, for example) can quickly escalate into a colic, something which is often fatal.
In other words, it's not because something is big that it's not fragile. So next time you get frustrated when your horse gets injured (we all do!), remember that it's the price to pay for these beautiful, big and fast animals.