This sentence amuses me as I am perusing through this technical article about 3D integration: Breaking a planar die into multiple smaller pieces will increase the yield of the smaller dice, since more candidate dice can fit in a given wafer. Because there are more candidate dice per wafer, the number of defects (which should be roughly constant), will effect a smaller percentage of the overall number of candidate dice.
It is as if saying the more children from the same parents, the defeat percentage in the children is less. Unfortunately, one defeated child is just so significant that it can't be just ignored by assuming as a defeated die. Just a random thought of my queer mind while reading reading reading...