How does Michael justify thinking of Hanna as a horse in Chapter 14?

At first Michael explains he thinks Hanna's physical characteristics remind him of a horse; she is strong and toned, yet smooth and soft, and her muscles twitch. He tells her that the comparison is not meant to be literal, as though she has a horse's face or mouth, but that it implies she has a certain strength, rather than delicacy, about her as a person.