You can approximate the length of any path (including circles) by adding the lengths of many small line segments that follow that path. Making a line segment bigger by some factor, will increase it’s length by the same factor. Therefore, scaling the circle by any factor, increases it’s circumference by the same factor. Scaling a circle is just scaling it’s radius so: Scaling the radius by some factor, changes the circumference by the same factor. That means the ratio between radius and circumference is always constant.

I hope this is decipherable :D