So here's female harpies using the current randomization system. All of these different variables, hair colour, face, body, and breast size, will all be part of the planned "DNA" system. In essence all of the possibilities for each of these variables will exist on a 0-1 scale (hair and trim colour being 3 0-1 scales, R G B) this value is stored within each monster to save their appearance, but also to pass on these values when breeding occurs.
When two monsters breed each variable will pick a parent and set them as the dominant genetic partner for that value. The value will then be "mutated" slightly (randomly shift away from the given value) the result is the child's value for that variable.
Breast size and penis size will be contained in each monster, even if they don't have that genital. Think of these as recessive genes. So, for example, let's say you mate two harpies, one with giant boobs. The child may take that trait from the mother, and end up with slightly larger, or slightly smaller breasts than her. However, it is also possible for her to take this trait from her father, since breast size is not displayed on males you can't know what value they'll be given. The male could have a recessive breast size of anywhere from 0-1, if he has a large recessive breast size gene, then the child in this case will be guarenteed to be born with large breasts. However, if the males recessive breast size gene is small, it could be seen as a 50/50 shot, since the child could pull this trait from either parent.