Answer
Being small when male and large when female could be adaptive if the role of the male is largely to provide sex cells, as in some spiders.
Work Step by Step
Try scanning through the chapter, and perhaps the rest of the book, to see examples of species with large or small males or females to get ideas for why this might be. Basically, what you come up with must include an explanation of why your idea makes the individuals more evolutionarily fit, better able to contribute to the next generation.