If you look at the your drum brake setup you will see it had no anchor in the bottom where the shoes come together as it just has the adjuster and the spring to hold the shoes together. In other words you have nothing at the bottom of the backing plate to hold the shoes from moving on the backing plate and thats why the brakes shoes have the longer lining on the rear shoes. Its because when you step on the brakes when driving down the road the shoes both push out but at the bottom they want to rotate and follow the rotation of the drum so the shoes try to move toward the back of the car as much as they can but the top has an anchor so the shoes sorta pivot at the bottom and this puts most of the braking on the rear shoe which is pushing out at the top by the wheel cyl and trying to rotate at the bottom towards the rear and the rear shoe takes most of the braking force.

Thats the main reason the rear shoe is longer then the front and if you know that it helps alot of times when you forget which way the shoes go as I need to think that way when I forget which goes to the rear as understanding why helps me remember. If you look at many newer cars or any cars where the shoes have a locked pivot point at the top and bottom so the shoes cant rotate then the shoes will usually be the same lenth as they both do alot of the braking. Anyway it usually helps me alot when working on cars when I understand why somethings are the way they are. Ron