This is an interesting question and could have several different answers:
- The Fairy Godmother who created the shoe incorrectly sized it. It's possible. She was old.
- The glass slipper, though referred to as "glass," may have actually been another see-through material that expanded (or stretched) over the course of the night when the heat from Cinderella's foot was applied to it. This makes sense because a true "glass slipper" more than likely would have cracked at some point in the evening, mauling her foot.
- As she danced, Cinderella's foot got sweaty, providing lubrication, and while running her fit simply came out. She was likely barefoot in the slipper, so this makes sense.
- The Fairy Godmother designed the shoe to come off at the critical moment. In her wisdom it was planned all along so that the prince would have something to track her by.
- Cinderella may have wanted to lose the shoe herself as a way for the prince to find her.
These are my guesses.
Thanks. I guess last point is the best. HMM.. CLEVER CINDERELLA.