One great 'Harry Potter Muscle Growth Fanfic' could be 'Harry's New Physique'. It shows how Harry discovers a magical way to build his muscles during his adventures at Hogwarts. It adds an interesting twist to the original story.
Another interesting one is 'The Muscular Transformation of Hogwarts Students'. This fanfic delves deeper into how muscle growth affects the students' lives at Hogwarts. It explores how their relationships change, both with each other and with their teachers. It also shows how they adapt to their new physical abilities while still dealing with the regular challenges of being a wizard or witch at Hogwarts. The detailed descriptions of the muscle growth process and its consequences make it a captivating read for fans of the Harry Potter universe.
There's also 'Muscular Magic in Hogwarts'. In this fanfic, not only Harry but also some of his friends start experiencing unusual muscle growth due to a spell gone wrong. It's full of humorous situations as they try to deal with their new, more muscular selves. The author does a great job of integrating the muscle growth concept into the magical world of Harry Potter.