"Come on, 'Mione.... Magic'll only advance with experiment, and practice."
Dusting off her uniform, Harry brushed against her bust rather more vigorously than she intended, gasping at the intensity she hadn't expected.
Hermione quickly wiped the smile from her face, "That body, is real... not just a disguise, even if you can't trick the stairs, you are fully, physically at least, a Gryffindor girl."
Harry couldn't meet Hermione's face, and shifted to patting the marks from her skirt, the wide circles of stone dust marking like a bloated capital M, where she'd slid back to the common room.
Even now, taking in the size of the mark on her skirt, Harry felt a twinge of sympathy, of empathy for the girls with large asses. But as she turned her attention back to her friend, it was easy to read the thoughtful expression on Hermione's face. It was pretty much her default expression.
"You could, you know, make it a proximity spell, that'd be safer." Harry suggested. "After all, once I'm up the stairs, it's too late to turn into a slide, ...."
"... besides where's the fun in just being another girl, in the girls' dorm." Hermione completed his thought.
With a sigh, Hermione agreed. Although, as she did, she realised there was something simple, something non-magical that they could have tried. Just going up the stairs together, or hand in hand. Would that be enough to fool the stairs, or would it send them both down on their asses.
However, Hermione wasn't humourless, even if her sense of humour was more subtle than Ron's.
Femini Totalus.
She tapped Harry on the tip of her nose with her wand. Harry blinked twice, as Hermione lowered her wand. In a moment, the change was clear, subtle but clear.
Harriet replaced Harry, her bearing more upright, more aware of her stance, her pose.
"Shall we?" Hermione gestured towards the stairs, and the girls ascended.
Harriet didn't expect the stairs to transform, and they didn't. Hermione was more cautious, placing her feet firmly on each step, but it had worked. The girls went to their dorm together.