This girl had pissed off this giantess and she had to get back at her. She had to make sure that she did it in such a way that she would never forget what the mistress did to her. That is why she had to find a way to know how to be a giantess and then use that to torture her. She did that and she cruelly turned her into a tiny woman and she had fun degrading and humiliating her for fun.