A third reason is historical. About 5 million years ago, the African forests began to shrink, and the grassland expanded. There were not enough habitat for the apes, and they fought each other for control. The losers (our ancestors) lost because they were not strong enough. These losers had to leave the forest (or be killed) to find a place to live. Most of these losers died of starvation because there is very little for an ape to eat in the grasslands, since apes cannot digest grass, the most abundant vegetation in a grassland.
So, we started out as the descendants of weak apes who could not defend a piece of territory in the forest. Since then we have evolved to be even weaker because our ancestors evolved to walk on 2 legs, and did not use their arms to walk around. Their jaw muscles also got weaker because to increase the size of the brain, a powerful muscle that is attached to the skull had to be disabled to make the skull larger. We needed the brains more than the muscles because our ancestors needed brains to figure out where to find food. Muscles won't help them find food because not even chimps are strong enough to bring down a zebra or a buffalo. OTOH, humans, by inventing spears, bows, and arrows, are able to kill large animals that even the stronger chimp cannot kill. You can say that we more than made up for our lack of brawn using our brains.
Humans have a built in mechanism which prevents their muscles from becoming too strong. Muscle cells give off a substance called myostatin. The bigger the muscles get, the more myostatin that is produced. Myostation inhibits the further growth of muscles. So, as someone exercises, their muscles get bigger, but the bigger the muscles get, the less effect there is from further exercise. This limits how big muscles can grow.
This is necessary for humans because of the importance of the human brain. If the muscles get big enough, their demand for nutrients and oxygen results in less resources for the brain and the brain function is affected, so the presence of myostatin is needed so that humans get maximum use of their brains.
I'm a layman but what I write bellow, kind of explains it.
Your question comes from the common misconception that "bigger" (hypertrophy) has to mean "stronger" but, besides that not always being the case in humans, it's even less the case when it comes to differences in strength between different species. Hypertrophy (or increase in muscle size) is more notable when you take an untrained individual and you train it. But training and evolution are 2 very different processes (the 1st one being much less efficient). The keyword is efficiency. Muscles don't need to be huge to be incredibly strong. That's what's confusing you. We only get that idea because, when we think about "stronger", we think about the untrained human body getting stronger by training. But reaching the maximum potential of your genes (training) is a different thing from already having different genes that have instructions for strong muscles at default level (untrained) and, that's where the difference resides. When a strength gain is a product of genetic evolution in a species, hypertrophy (or increase in muscle size) is only one of the ways that strength increases. Millions and millions of years of evolution allow especially for energy-efficient strength gains that do not involve hypertrophy (or volume increase)
But even within the human species, some people will have more potential for strength and less potential for hypertrophy (muscle volume increase). I know a guy who bench presses 240 lbs and, when you look at him, he's just a skinny guy. And lots of muscular guys have trouble bench pressing 240 lbs. I've seen huge guys who are not stronger than me and I'm not particularly strong. All that size does not necessarily mean strength. If you observe olimpic heavy-weight lifters, some of them are very small and, yet, they are tremendously stronger than many guys double their sizes ;). In different species with even more different genes, hypertrophy is even less important.
I think it's just due to them having more powerful muscles than humans, and something else due to their genetics as well. I have found an article online for you to read about it to get more insight into it, it's great and informative on the subject. Have fun reading it! :)
