Sin and Cos are about the angles of the corners of the triangle other than the right angle, they are not about the length of the sides. Sin and Cos are really ratios - given a particular angle the ratio of the sides and the hypotenuse will always be the same (it's a fraction, or a decimal if you will, between 0 and 1). So squaring just the ratios and adding them together will always be 1 because for the ratio itself, the length does't matter.