Finally got around to finishing this one, decided I'd get it done for Ai's birthday today. I wanted to make sure I was in the right mindset to get the expansion parts right. Ended up writing a lot more than I expected, since I love adding a bunch of minute details and moments.

There's a time in every scientist's life where they get so caught up in trying to make something, that they forget if they even should, and Rina was having this moment right now. An idea had popped into her head that seemed just plausible enough, and each step of the process furthered her curiosity and belief, but now holding the finished concoction, Rina wasn't sure what to do now.

In her hand was a chemical formula that could deliver a calorie-bomb directed at the drinker's lower half; in basic terms, making their butt, hips, and thighs grow bigger. At least that's what it's supposed to do. Nothing like this had ever been made before, so Rina had no idea if it even worked. The only way to find out was to test it out.

Rina wasn't going to drink it herself. Placing a hand on her own behind, she felt like she was already a good size (perhaps even a tad too big, with her thigh highs already digging into her legs), but any bigger would look silly on her small frame. And if this formula worked too effectively, then she could grow so much she'd end up stuck with no way to undo it herself. Who would even want to submit themselves as a test subject though? “Rina-chan board, thinking...” Kasumi probably would, but just thinking of Kasumi bragging about her increased size every day was already annoying Rina. There was only one person who would trust Rina enough to help her out.

“I'm here Rinari~!” the cheerful voice of Ai Miyashita bounded through the club room as she entered. Rina told her gyaru best friend to meet her alone after school, so no matter the outcome of the experiment, no one else would be there to see. “I'm filled up on snacks like you asked me to, so what do you need?” Ai patted her toned stomach. It wasn't distended as someone like Emma's would have been if Rina asked the same from her, but Ai was feeling full.

“Ai-san, thank you for coming here. I needed help with an experiment, and testing this new serum I made.” Rina pulled out a water bottle from her bag, holding it out for the blonde girl to take.

“A new experiment? What is this supposed to do Rinari? Is it supposed to fix a stomach ache or something?”

Rina blinked at Ai with her usual blank expression for a moment. It would have been easy for her to lie about the true purpose to one of the other members, but to her best friend, Rina had to tell the truth. “This... this is supposed to make your butt bigger.”

Ai in return blinked at Rina with her own blank expression, before instinctively moving her hand behind as if to protect her butt. “Eh!? Make my butt bigger!? Why would I want that? Why would you want that!?” Did I even hear her right? Ai wondered to herself.

“Wait! I know it sounds strange!” Rina threw her hands in front and waved them at Ai, “But please Ai-san, you're the only one I can ask for this! I'm sorry, but please help me out!” Rina bowed to Ai, hoping her exaggerated movements could convey the sincerity that her face couldn't.

The two stood silent for a moment, as Ai's mind ran in circles wondering if she really should do this. She already had the highest waist-hip ratio out of the club members, so anything more would just make her proportions more extreme. The hand behind Ai's back protecting her bottom changed roles, as it inquisitively cupped one of her butt cheeks and gave it a lift. She had a nice butt, but it was mainly her hips that contributed the most to her measurements. The one with the biggest ass in the club was her duo partner in Diver Diva, Karin Asaka. Ai had seen that ass many times during their practice sessions, and she always felt a tinge of jealousy because of it. Why did Karin get to have a big butt and big boobs? Just a few more centimeters and Ai could take the lead, and let her come on top over her friend/rival...

Ai broke the silence, “Alright, I'll do it,” as she unscrewed the cap of the bottle. “So I just have to drink it, that's all?”

Relieved that things were under way, Rina enthusiastically replied, “Yes! It does require energy for the growth, but I already had you take care of that before coming here.”

Ai chuckled. “I ate a bunch of candy that was getting old, so it's better if all that sugar went to my hips instead of elsewhere. It would have gone to waste, or to my waist! He he, get it?” her bright grin came out, free of any of her previous hesitations.

Rina held up her notebook with a smiling face drawn on the page. “Rina-chan board, nice one!” she gave with a thumbs up. Flipping to a new page of the notebook, Rina wrote down some preliminary notes before producing a tape measure. “Let's get your initial measurements first” Rina said as she knelt down to put the tape around Ai's hips.

“Oh, no need for that! I can tell you it's 86-”

“87 centimeters” came the response that cut Ai off.

“Huh? No way!” Ai felt a little embarrassed that she didn't even know her butt was already bigger. “Ahaha, I guess I grew recently... but don't worry! I'll still drink this!” Ai held the bottle up to her lips, hesitant to start drinking. “Um, ready whenever you are...”

With pen to paper ready to take notes, Rina nodded at Ai. Putting her lips on the rim of the bottle, Ai tipped it up and started to drink. Gulp, gulp, gulp... half of the bottle was downed before the fullness in her stomach stopped her. “Bleh, you could have added some flavor to this,” sticking out her tongue in mild disgust, “a flavor that tastes good and not like chemicals.” But there was no time to worry about the taste, as Ai turned her head around to watch behind her, sticking her butt out to get a better view.

When nothing happened at first, Ai was ready to call the experiment a failure. But then she could feel the fabric of her skirt slowly moving, like it was being pulled up. One hand shot down onto one of her butt cheeks, then the other just as fast to double check. “Rinari! I think it's working!” Ai spun around to point her ass towards Rina, pressing her skirt down with her hands as taut as she could. “My butt, hips, thighs, they all feel tingly now!”

Rina observed as the mass underneath the skirt bulged out more as each second passed, filling out more of the skirt and almost starting to hang over Ai's fingers. Sure enough, Ai's hips and thighs were getting growing as well, making the already curvy gal even curvier.

“Ooh, this is getting exciting!” Ai exclaimed as she bounced on her toes, making her growing ass bounce in return. “It feels so jiggly now, I can really get behind this!” Another giggle escaped Ai's mouth, but it was clear what was making her happier right now.

“I need a more direct view” warned Rina shortly before flipping Ai's skirt up, exposing Ai's rear end.

“Hyaah! Rinari! Is this really necessary!?” a new blush joined Ai's smile as she held her skirt up, still letting Rina see her full butt. Her ass was clearly the largest in the school now, let alone the club. Her hips and volleyball sized cheeks were wide enough to overflow the club's chairs, and the overstretched panties that Ai was trying to fish out of her butt crack didn't have much time left to cover what little it was right now. The softness and the weight brought the mass down, giving Ai's butt more of a fat look than the perky round shape it had originally.

Rina wanted to measure Ai again while she was mid-growth, but it looked so soft... touching it was part of the experiment observations, right? Rina's hands reached out and pressed into the soft flesh, her fingers sinking in and causing Ai to yelp again. Rina's hands explored the test's results, kneading the fat on Ai's ass and feeling as the growth pushed past her fingers. Just part of the experiment, all part of the experiment...

“R-Rinari!” Ai finally found the urgency to call out to Rina. “I-Isn't that enough touching?”

“Ah, s-sorry Ai-san...” Snapped out of her trance, Rina took the tape measure and looped it around Ai's ankles first, as there was no way she would reach around Ai's circumference, before bringing it up to her huge butt. But getting a stable measurement wasn't possible, as Ai's ass was growing fast enough now that the tape kept digging in whenever Rina gave it more slack. “140, 142, 145 centimeters...”

“O-one hundred forty-five!? And it's still growing!?” Grabbing her own wobbly ass, Ai adjusted her legs to better handle the new weight on them, rubbing her thighs against each other that had filled out the thigh gap she had before. Now well past any comparisons to sports balls, Ai needed two chairs to handle her fat round ass. Ai had already unhooked her skirt and pulled it over her head, since there was no way she'd pull it down and over her massive mounds. The last lower garment Ai had on was on it's last threads, as her panties were stretched further and further until snap!, they finally snapped apart. Rina would have taken them to analyze how they managed to stay on for so long. Ai's hands darted in front to protect her modesty, since her huge butt covered everything from behind.

Not to long after Ai felt the tingly feeling start to fade. She brought a hand behind to press onto her shelf of her ass, to test if it was still growing.

“I think it's done growing, Rinari... I didn't think I'd get this much junk in the trunk...” allowing herself to feel her new body, Ai squeezed and fondled herself, shaking her hips to see how her butt swayed and bounced around. Her ass was absolutely massive, with ridiculously wide hips above equally thick thighs. She was wider than three of her past self standing side by side, a past that was only 30 minutes ago. ”Geez, I'm so huge... but it feels nice having it this heavy and bouncy...” Having to stifle her own moans, Ai let go of her butt after one final lift, letting her cheeks slap down against each other and her thighs.

“Rinari... it looks like your formula was a success... a BIG success...”

“Yes it was, Ai-san...” Rina was nearly too in awe to talk. “I could make another formula to reverse the effects, would you want that?”

Ai felt herself up once again, biting her lip. “No, I think I'll stay like this for a while... Getting my outfits and clothes resized shouldn't be a pain in the ass!” Ai followed her pun with another laugh, and a slap that sent waves across her cheeks. “But could you help cover me up and get me home? In case I grow any further, I want to get out of here while I can still fit through the door, even if I have to shuffle sideways through it.” Ai glanced at Rina, giving a wink. “I'll let you have some more physical examinations as a reward~”

Rina couldn't have agreed faster.

“Urgh, why'd I have to eat so much last night...”

Walking into the club room in the morning, Kasumi Nakasu was rubbing her upset belly, when she spotted a half-empty bottle left on the table.

“Huh, what's this?” Kasumi asked herself, picking up the bottle. “This looks like one of Rinako's drinks... It's probably some kind of performance booster... Ni hi hi hi, you shouldn't leave your secrets out like this Rinako! Let's see what kind of performance growth this will give me!”