Meg seemed excited that you were shorter. Let's test that out.

As you lay in bed trying to fall asleep and mulling over the day, you couldn't get the image of Meg's face out of your mind when she saw your shorter height while measuring you. You could swear you saw a glimer of excitement, perhaps even lust.

Sure you knew it was a long shot that she shared the same fantasies as you, but if there was even a possibility that she was in to the idea of you shrinking, you had to investigate it. Plus, if it turned out you were wrong you could probably just undo it with the stone.

The question was, how should you make yourself shrink? Should you change reality for everyone except Meg? Or perhaps link it to her own growth, maybe every inch she gains must be taken from you somehow. You mulled it over, taking inspiration from all your favorite size-change stories.

Eventually you made a decision, and grasped the stone.