Daikanojo

Hikaru Mitsushima (15) is a run-of-the-mill freshman who holds an unrequited love towards her senpai (a sophomore) from the astronomy club. But one day, after she makes a wish upon a falling star, she becomes fused with the star (alien) for some reason. Hence, she obtains the strange ability to grow into a giant. That is when Hikaru's life goes for a complete change, as she begins to do battle as a giant heroine?!

(Source: Shogakukan Creative, translated)