text: There is a relationship between the speed of rotation and how much the object is flattened. The outer edge of the galaxy is moving a a very high rate of speed. Our own solar system is moving at 200 kilometers per second as the galaxy rotates . But Earth is only rotating at around 1600 kilometers per hour, or roughly 0.5km/second, and it is, in fact, bigger at the equator than elsewhere, not a true sphere. It's 6357km radius at the poles, and 6378km at the equator. The spinning makes it oblate. This is true of all the spinning planets. The faster they spin, the flatter they are, overcoming the gravitational attraction.