Being small when female and large when male might be adaptive if the male is supposed to provide a large reward for the female or provide protection for her. The opposite could evolve when the role of the male is reduced largely to providing sex cells, as in some spiders.
Work Step by Step
For this question, try to imagine when small or large size would make a male or female more successful at reproducing, making that state adaptive. You might flip through the chapter for ideas, or even through other parts of the book.