Because the functions are a ratio of two side lengths, they always produce the same result for a given angle, regardless of the size of the triangle.
In the figure on the left, drag the point C. The triangle will adjust to keep the angle C at 30°. Note how the ratio of the opposite side to the hypotenuse does not change, even though their lengths do. Because of that, the sine of 30° does not vary either. It is always 0.5…