I was inspired by a little unfinished piece I had on writing.com. After planting the magic beans, the Cutie Mark Crusaders climb the resulting beanstalk to find themselves right on the doorstep of the Princesses' palace, but they're smaller than the mares' hooves. Upon finding the tiny fillies Celestia carefully asks what led them to this, which leads her to realise that Luna must've set the matter up and, upon confronting her, listens to the passionate response that this was the only solution Luna could find to address her personal desire for her own pet.

Background: https://www.furaffinity.net/view/52074523/
Characters: https://www.deviantart.com/fruft/art/Luna-and-Celestia-emotionally-talking-Vector-810863418 and https://www.deviantart.com/qazwsx302/art/Shocked-Cutie-Mark-Crusaders-567177783.