All circles are similar, so for every angle measure, the "sides" or lengths of the circle will be proportional. Arc length, s, and radius, r, ("sides" of the circle) will be proportional, since a circle of any radius can be dilated and translated to map onto a circle with any other radius. Since these are similarity transformations, all circles are similar and the value of (s / r) or the radian measure for every angle measure will be the same.