Humans have a unique ability. Unlike any other living thing that we know of today, we are the only species with the ability to change sizes. As in physically change sizes with just our will. In fact, size changing is our adaptation since the beginning of mankind hundreds of thousands of years ago. This helped us survive and become wise enough to create civilization. Despite this, science has yet to explain why mankind evolved size changing abilities, but there’s no doubt it has been very useful to our survival. However it can also be a liability.
The one thing that many would first notice is the extreme sexual dimorphism. The female sex is almost twice the height of males. Girls are born at an average of 5.5 pounds while boys are at a measly 2.2 pounds. The growth rate is exponentially faster and lasts longer than boys of the same age. Already at a size advantage at birth, girls will grow to an average 6’3’’ in their tweens and then a huge growth spurt to over 7’5’’ average by later teens. Boys don’t have any growth spurt, so they would grow slowly and only reach an average of 4’0’’ by their max growth. This growth, however, is a scaled up type of growth. What this means is that the proportions of a person remains relatively the same when grown (as in no stretched body parts or thicker bones to support weight and other biological factors). This size discrepancy naturally grants the woman to be the dominant sex over the men, but this has changed in recent times since the “Near Crush” event had happened. Nowadays, male and females have developed a gentle and sometimes even a familial bond towards each other. Even more surprising was that it didn’t even happen through any forced means. The event and the change in relationship will be explained more in details in another set of lore.
Out of all known living things on this planet, no creature could change sizes at will and break most physics as the human species. For some unknown reason, only mankind has evolved this trait, which is like a superpower for everyone. It does have its limits though. Females can only grow larger and shrink back to their usual height while males can only shrink smaller and grow back to their usual height. There are very rare instances where a man can grow larger and a female can shrink, but this is irrelevant in terms of statistics. Size changing is also closely linked to emotions and mental state. A sudden or extreme emotion can cause a growth up or shrunken down of a few inches. Size changing can even occur subconsciously and affect the person without them even knowing it. This is why starting in preschool, ways to control sizes efficiently is a core curriculum. Again, scientists debate the true intentions of evolution to come up with an ability to be tied to emotions. A popular theory theorizes that it can be used as a defensive mechanism for women to overpower larger threats while men will use their smaller size to either flee or collectively overwhelm them. Whatever the case, this is where you can say it is a liability in the modern world now that we don’t live in the wild anymore.
Note that I have said break most physics and not all. The heaviest violation is the Square-Cube Law, wherein as an object grows in size, its volume grows faster than its surface area. The opposite is true for shrinkers. This law of physics does not apply in our world as all human proportions remain the same when growing or shrinking, just increased or decreased. However, that is to the extent that size changing has on the Laws of Physics. As a woman gets larger, she becomes slower, her voice deeper, and requires more energy to retain any height pass her original size. Of course she will also be much stronger the bigger she is. The maximum size reached was the size of a small country, and that was during the “Near Crush” event. Males really have no limit to how small they get. However most have sense in not going towards the microscopic size unless on their own risk or as a job. Men will be faster, become more durable, and see and hear the world slower the smaller they go. But they are not protected from loud sounds and any normal sized objects that obstruct or even kill them. And maybe of course the embarrassing nature of a higher pitched or inaudible voice to the normal-sized human ear.