Toriel sat and sighed, alone. It had been only a few days since the last human child to fall into her home had left her, and she missed them terribly. She worried for their safety, not knowing whether or not they were surviving alright in the harsh world of the underground. It was incredibly depressing, and not even her favorite food could cheer her up.
Trying to find something to do, she went over her bookshelf once more to try and find something of interest, knowing full well she'd read every single one of these books at least a hundred times over. Finding nothing that she felt like reading, she sighed again and went out for a walk. Maybe she'd do some shopping? It was better than nothing at least...
A short time later, Toriel was returning home with a slightly brighter mood. Setting down her shopping bags, she pulled out a new book to add to her collection: a spell book, to be more specific. Sure, she knew fire magic and some basic cleaning spells, but something about this book of body molding magic just really caught her eye. After putting away her groceries, she settled into her favorite chair and opened the book, beginning to read it over. It was a fascinating idea, being able to reshape a monster's form.
As she read, her mind began to wander and daydream. Perhaps she could use this magic to help her human visitors... Make herself so beautiful that young men would want to stay with her forever, or make herself so strong that she could protect her children no matter what... She bit her lip at the thoughts, her lonely mind finding these fantasies incredibly appealing.
Then she stumbled upon a page that snapped her out of her haze, finding the section on overall size. Feeling impulsive, Toriel went over the spell for growth, deciding to try it out right here and now.
Of course, in her excitement, she hadn't read thoroughly to see the possible side effects, which included (but were not limited to) extra growth in certain parts, intense hunger, and a loss of empathy for smaller creatures. Ignorant to the risks, Toriel cast the growth spell on herself, just wanting to be a little bigger.