The stationary jaw gets more of a rubbing and gouging action against it than does the moving jaw. This is due to the moving jaw's tendency to push the crushed material against the immobile frame side of the crusher. This same rock will roll somewhat against the moving jaw, resulting in lesser rubbing or gouging action.
Typical wear ratios between the stationary side and the moving side are 2:1 stationary wear. This higher wear rate on the fixed side is true with cone crushers as well, though there we've found that the ratio of bowl liner to mantle wear normally runs between 1.2 and 1.5:1.
In both types of rock crushers you'll see design exaggerations to the fixed side in an effort to even out those ratios. That's so that service related maintenance can be carried out at the same time to both sides of the crusher.