Smaller shoe always goes toward the front when you have a movable lower anchor, (star wheel between the two shoes). The front shoe tries to grab the drum and rotate with it. That action pushes the bottom of the larger rear shoe into the drum. The piston in the wheel cylinder also pushes the top of the rear shoe into the drum. That's why they call it a "duo-servo" drum brake. Two forces are acting on the rear shoe. It's bigger because it does all of the stopping when driving forward.