The idea of differential reproductive success (DRS) is to determine the success of a pair of offspring to a certain size.
What makes DRS work, as well as this blog post, is that it’s a little bit weird: It’s not the average, it’s a range. Most animals lay eggs at a size that is about 2 standard deviations (SD) larger than the average. To put this into perspective, the two SDs on the height of a human are 3.4 ft and 5.5 ft.