You ask if she's sure she wants to go through with this. Cassie's head bobs up and down, either agreeing with you, trying to bounce her butt more, or that's the way her orgasm made her move. Whatever the reason, it's good enough.
"Your ass will grow until you can barely walk. Your butt will be bouncy, soft yet firm."
Nothing seems to happen. Her breathing slows down, she stops shivering from orgasms. Her eyes even come back into focus long enough to look at you and whisper: "Oh... oh fu-"
"FUUUUCK!" Her butt grows explosively, spreading out behind her like a dress. It defies gravity, bouncing up and down as she grabs a handful and her clit. Cassie rubs both, moaning before she gives up on her clit entirely, her massive bum seeming to be the most sensitive part of her now.
In ten seconds, half the office is devoured by booty flesh. It stops with her hips at a mild five feet across, and three feet of round booty jiggling off her backside. Thighs thicker than your body hold her for a moment, before she comes again and falls with a wet splat onto a pool of her own pussy juice.
Cassie leans back on her ass cheeks like beanbags, still squeezing's her bum. Blearily, she looks up at you. She strokes her thigh, trying to be enticing but ends up cumming again. "Do yo... do you.. like..."
"Please... touch me... I'm so horny, and I... I want you to touch me... Please have... s-sex... or keep making me bigger... Just.. don't leave yet..." Cassie's crying, trying to move toward you. It's a little sadder than you imagined. "I wanna know you still... like a massive girl who... can't stop cumming..."
She sniffles, trying to waddle toward you. Since she can only walk with her butt, you're not sure she can stand up on her own. At least you're the one closer to the door, so her ass didn't trap you in here. You decide to...