It could also be a way to explore themes of self - esteem and acceptance. Percy may struggle with feeling less important or capable because of his size. Through his journey in the fanfiction, he could learn to accept himself and find new ways to contribute. And Annabeth could help him see that his value doesn't depend on his physical stature, strengthening their bond in the process.
It could be a symbol of vulnerability. Percy is often seen as the hero, but being smaller makes him seem more fragile. This can add depth to his character as he has to overcome not only the usual challenges but also the limitations of his new size. It also gives Annabeth a chance to be more of a protector, which is a role reversal that can be interesting to explore.