As soon as your girlfriend spoke those words, you could feel a surge of energy flow through the room that made the hairs on the back of your neck stand up. You could hear her catch her breath as her eyes rolled back.
"Oh wow, this feels...incredible!"
The first thing you noticed was her body beginning to...stretch? You watched in amazement as your short girlfriend rapidly grew to match your own taller-than-average height. Whatever she was becoming, at least you would no longer need to worry about bending down so far when you kissed her.
The transformation coursing through her body didn't seem to be causing her any pain - very much the opposite. She quietly moaned as her hands roamed over her changing form. Somehow the denim shorts and t-shirt she was wearing were still intact, but since they were sized for your girlfriend's old body they now revealed a lot more of her new form. A big smile spread over her face as she saw your questioning look.
"I know you always tell me how much you adore my tiny and petite body, but I've seen how you ogle all of those strong and fit girls at the gym. And...oh wow that feels good...I've always kinda wanted to know what it's like to be an athletic tomboy."
She can tell by the huge grin on your face that she made the right choice.
Her moaning intensifies as her legs begin to tone up, with calves and thighs becoming more defined while her ass becomes firm and sculptured. You briefly thought that this girl must never skip leg day.
With the changes moving up to her torso her tummy lost the slight muffin top that she's always had, flattening out to reveal some tempting abs. Arms and shoulders swelled as she gained some muscles there, and even her neck expanded noticeably. Your girlfriend's breasts - always pretty large on her petite frame - visibly shrank but her t-shirt left you unsure about exactly how much.
Her face lost any baby fat that she had and became more angular with her lips deflating slightly. She winced as her nose grew a bit larger, gaining a slight crook that must have been from an injury in her new self's past. Her long blonde hair, that had been down to mid-back on her old body, began to shorten while at the same time darkening to a deep brunette and straightening out her natural curls. It finally ended up dangling barely to the top of her neck, taking on a less styled and more unkempt look.
Finally, her clothes began to shift to accommodate your girlfriend's new athletic form. The sandals on her now-larger feet wrapped around, enclosing them in a pair of sporty navy blue trainers as the tops of some white no-show socks peeked above them. Her denim shorts (now extremely short and tight on her new legs) loosened and lengthened slightly, the cotton material giving way to a silky smooth polyester sheen that took on a bright sky blue color with a wide white stripe running down each side. The tight t-shirt also loosened, with the sleeves receding leaving her clad in a crisp white tank top that ended right above her belly button giving you a teasing view of her new tummy. The lacy black straps of her bra widened and smoothed out, becoming what must be a stretchy sports bra in a sky blue color to match her new shorts.
Breathing heavily, as if she just finished an intense workout, the energy subsided leaving your transformed girlfriend in her new body. As she spun around to admire herself in the room's full length mirror, you were stunned by the changes. The girl you now shared the room with looks like she could be your girlfriend's sporty, tomboy sister. Flexing her admittedly impressive arms in the mirror, she makes eye contact with you and asks in a slightly deeper and much more confident voice:
"So what do you think?"