Mei Hatsume had asked Momo to watch over and take care of her “babies” for her while. After touching one of her inventions Momo is shrunken and is unable to reverse it, so she goes out looking to find someone to help her.
Eventually, she encounters the gigantic Tsuyu Asui. She tries to get her attention but unfortunately at Momo’s size, she looked like a tiny bug causing Tsuyu to decide to squish her beneath her foot.