Originally Posted by Andrew R Stewart

Thinner spokes have more elasticity and will stretch more for the same tension. Thus as the rim is compressed in to the hub (from the weight and ground contact), or as the wheel is flexed laterally, the thinner spokes will be less likely to loose all their tension and their nipples less likely to unscrew bit by bit. This same principle is why many wheel builders like to use butted spokes.

This is a method that is as old as tensioned spokes are. Andy