It's traditional to set up a rear brake with a longer straddle cable (less mechanical advantage), and shimano stock link wire lengths reflect this.
(my) Reasoning is less mechanical advantage allows the brake pads to be set further away from the rim, as a rear rim is more likely to be or go out of true and will flex more climbing/sprinting.
The rear brake also has far less braking to do because of weight transfer, and will just skid while the front is still stopping you. It's more useful for scrubbing speed especially in corners and less mechanical advantage gives better modulation.

I think the shimano link wires are good and fine tuning mechanical advantage is rarely the real issue with poor cantis. The advantages they have over traditional straddle cables is they already have the wire pulled taunt, so they brake immediately, while traditional straddle cables take some lever travel to pull the straddle cable into the v-shape. And, the reason they exist is they catch a broken brake cable so they are safer unless you run a doodad (not really relevant unless you're underbiking).

I am just rambling and you simply need new pads.