Your step-mom confronts you about your browsing history. She demands an explanation for all the searches about “growth,” “clothes ripping,” and giantesses. As you describe the idea, she tries (and fails) to hide how turned on she is getting.
Eventually she admits she’s obsessed with the thought of outgrowing everything.
You reveal you have an untested formula that might actually make it real. She doesn’t hesitate, she demands it and has the entire thing.
The effects hit fast. She becomes extremely aroused, starts touching herself, and moans about how good it feels. Her body begins expanding. Clothes stretch, strain, then rip and shred as she grows. She tears through most of her outfit, leaving only scraps, while her size rapidly increases.
She keeps growing, soon pressing against the ceiling. Horny expressions, constant masturbation, and increasingly power-hungry dirty talk fill the scene as she describes how addictive the growth feels and how much bigger she needs to become. She bursts out of the house and continues expanding, houses, streets, cities, and eventually the entire world disappearing beneath her.
She reaches planetary size and orgasms hard from the sheer scale of it. The final shot is her still growing, eyes rolling back, repeatedly moaning the word “bigger…”