Probably closer to 20,000 words. Just a story involving someone's annoying little sister going through a bizarre growth spurt. She is someone who is a skinny, borderline aspie weeb and often follows around the mc, who is himself a nerd. The teenage girl inexplicably begins to grow larger, even though her proportions remain the same.
She increasingly becomes a massive sensation as she grows, even with her weird quirks. She also obessesses on her own size, and there could be a focus on descriptions of how small places are for her, how much she eats, etc.. Throughout this process, she gets spoiled/demanding with people around her, including the mc. In turn, he becomes obsessed with her, which he tries to suppress.
After he admits his obsession with her, she does all kind of lewd things to him, including sex as you can call it with the size difference. Thinking around 11-12 feet tall here. Open ending with her still growing. She still loves him, but she only spends so much with him.
I am trying to leave it mostly open beyond that.