I’ve been catching up on Bloomin’ Faeries and found a breast expansion story arc that begins with this comic. Princess Heather’s younger brother is able to achieve a kind of mind-control over her that yields to total body control, and he commands her breasts to grow. And boy, do they! Soon her perky C-cups are so big she can barely stand up!
There’s quite a bit of gender, body-swapping, age changing and general transformation in the comic. (There’s also a lot of sex.)