So if we increase this by, say, two, this increases by two and these guys cancel out. So no matter what, we increase this by, say by three, by four, by five, by six and so on, this will increase by a similar increment. Meaning that you could cancel these guys out and you get the same ratio over and over and over. So pressure always stays the same no matter of size. Density stays the same for the same reason. Because density is mass over volume, it's also a ratio of two extensive properties.