So, how will the fiery redhead start sprouting more breasts? In order to keep within the rules of the story, Hogwarts won't start admitting students until their sixteenth birthday, making it more Hogwarts College for Witchcraft and Wizardry, rather than just a simple school.
The Chamber of Breasts:
Following the events of the Chamber of Secrets, Ginny enters her first year of Hogwarts with excitement in her heart, and a mysterious, blank book in her possession. Rather than disposing of the unusual book, she begins to use it as a diary of sorts, scribbling down all sorts of thoughts she has about Hogwarts, her lessons, and the people around her.
What she doesn't realise is that with each new entry, the book steadily begins to change her body and twist her mind. At first, Ginny notices that her chest seems a little fuller than usual. But as time passes, she realises that, not only are her breasts swelling, but she's beginning to sprout more and more of them. And as much as she knows she should despise them and figure out how to get rid of them, she just finds it so... exciting!
And if her odd excitement at growing more breasts wasn't enough, the diary begins to write back to her, urging her to accept her changes and to keep jotting down her thoughts. Will Ginny be able to figure out the insidious intentions of the book, or will she end up as nothing more than a pile of bloated breasts, trapped in a forgotten chamber beneath an abandoned bathroom?
The Half-Blooded Prince's Mysterious Brew:
Taking place in Ginny's fifth year, her friend and crush, Harry Potter, seems to have become something of a Potions Prodigy overnight, despite having spent the last five years barely scraping through Professor Snape's subject.
One fateful day, Harry asks her if she wouldn't mind assisting her with an unknown recipe he found 'lying on the ground that he decided to scribble into his copy of Advanced Potion-Making'. Eager to spend time alone with him, Ginny excitedly agrees, paying no mind to the potential dangers of creating an unknown potion. The duo successfully brew the concoction and Ginny, after hearing Harry mention something about it being created for a redheaded girl, decides to drink it.
A few days pass, with Ginny finding no ill effects from the mysterious brew. One day, however, she finds a few oddities with her body. Her breasts are rounder, her belly and buttocks seem much more tender and sensitive, and odd bumps, not unlike nipples, begin to dot her body. Thinking little of it, she continues about her year, with the changes to her body becoming more and more noticeable with each passing day.
Or, perhaps, you want a different story?