The front shoe is called primary because acts first ( going forward ) when it contacts the drum it gets forced down which causes it to also push the secondary shoe (rear) through the self adjuster. The rear shoe applies the most force due to being pushed by the primary shoe, rotation of the drum and the anchor/pivot point being at the top. Due to this design thats why you should have more material on the rear shoe because it is doing more work than the front shoe.
Simples..
