The Growth Charm (incantation unknown) is a charm which increases the physical size of the target; it is unknown whether the spell is used to increase the size of the target's cells themselves, or if more cells are created to give an appearance of a greater size.
During his Charms O.W.L. in June of 1996, Harry Potter confused this spell's incantation for that of the Colour Change Charm. Thus, his rat swelled to the size of a badger instead of turning orange as he was supposed to have done to it.
Behind the scenes
Given that Harry confused the two spells, it is likely this and the Colour Change Charm have similar incantations.
- Harry Potter and the Order of the Phoenix (First appearance)
Notes and references