Originally Posted by 79pmooney

It doesn't. What causes much of the damage is the spoke going loose when the rim to hub distance shortens (as you go over a bump, say) and the spoke goes completely loose. So a lighter spoke must be stretched out more to achieve the same tension. The spoke that is stretched more will still have some tension after the rim-hub distance is shortened by a given amount instead to going fully loose.

At the point of build, you are right. The lighter spokes get tightened to exactly the same tension as heavier ones would. It is thousands of mile later that the lighter spoked wheel shines because those light left-side spokes have never gone loose while heavier ones might have gone loos momentarily tens of thousands of times (especially under a heavier rider).