To be exact, Amaterasu has no gender, but Ammy is hinted to be a female through the game. IMHO, 'she' can change gender whenever 'he' likes. Female the first second, male the other. You choose. (and actually, all of the gods are supposed to be genderless)
actually, the game does make hints that okami is a girl. First she is the recreation of Amaterasu(japanese goddess). Then the gods that re-unite with her calls her "mother". And even the shrine maiden dead husband saw okami true form(a female woman). Waka hints it as well.
A normal woman standing next to an another woman with large melons would look and be impressed. Espensaily if the normal woman have small breasts.
And i have the offical artwork book. And it is clearly said the Okami Amaterasu is a female.