Inherit, Imbue is a story written by TK Hoshikuzu. It ran in Yes, And 5 at http://www.shousetsubangbang.com/mirror/inherit-imbue/.
Coming-of-age story with lots of radioactive, somewhat grotesque magic.
I say 'radioactive' because as I wrote this I kept imagining the origin forest as radioactive and magic as the result. Fun fact, the idea for this started with Iyr and Nineka, the two significant-but-still-minor characters in the story.
Also, aren't you all tickled by the fact that women are stronger magic users because they have more fat? It does mean that fat, powerful witches exist, but they have to have the stamina to wield all that power too.