Gentle Giantesses

Hmm. I've got another idea:

James (Blake's father), after dying, he turned into a powerful angel, he watched his son from above and he could see how the little boy whose future had been seen by him was turning into real. After Joana's growth, Blaze's appeareance, and a little before the ending of the events of Shrunk With Your Girlfriend/GTS in GTS line, James sacrificed definitively his life to seal Blaze, and before dying he granted Joana the growth power. He knew that the world needed a protector and died. Blake began to train (he had kept his skills by only fighting for real) and the substance which caused Joana to grow made him stronger.

This idea would eliminate the one where Blake and Rick use the substance to make Joana grow again in (she already knew some).

I hope this would be useful to your story.