Modern drum brakes are "self energising", meaning that when applied, as the smaller primary shoe contacts the drum the whole assembly rotates around in the direction of travel. The anchor bolt at the top of the backing plate stops the movement and the larger secondary shoe is forced against the drum. So not only is the wheel cylinder pushing the shoes against the drum, but the force of rotation is multiplying the braking action. The secondary shoe is taking the brunt of this and that is why that shoe is larger. In reverse, the opposite occurs, but since little braking force is needed going backwards, the primary shoe can be smaller.
