Golf Ball's underground lab. Almost sunset. Tennis Ball walked down the cool metal staircase, his loud thumping footsteps echoing in the giant underground structure. Once he made it to the bottom, Golf Ball was there waiting for him.
"Ah, TB, you're here, finally," Golf Ball said, sounding annoyed. "Anyway. I want you to test out my latest invention!"
"Oh! Cool. What is it?" Tennis Ball said.
"It's a growth ray!" Golf Ball said, and Tennis Ball looked intrigued. "It makes anything you zap with it grow bigger. So far, it's... buggy at best. I haven't tried it on any living organisms, but the inanimate objects i've zapped with it have... not been very lucky. Hopefully after my recent adjustments it will work better!"
"O-oh." Tennis Ball started to get nervous. "Why don't you test it on yourself..?"
"Because if I used it on myself, my feet would be too big to be able to operate the gun and change myself back!" Golf Ball exclaimed.
"Oh. That actually does make sense." Tennis Ball said. "Wait, wh-"
"Before you ask, yes, it's also because I would rather have the ray set /you/ on fire rather than me." Golf Ball said, retrieving the device from a nearby drawer. "Ready?"
"W-wait, GB, b-before you-" Tennis Ball braced himself as he heard a loud BZZT noise, and rolled back, blinking. "Oh. It didn't work, I guess-" Right as he said this, he noticed a strange electric sensation tingling his entire body, starting at his feet, as he slowly started to grow bigger. "Oh-! It's working-!" Tennis Ball, still in a sitting position, watched as his feet continued to grow bigger, much to the bewildered excitement of Golf Ball.
"YES! Finally, it works!" Golf Ball cheered, which quickly changed into mild panic when Tennis Ball continued growing with no sign of stopping. He was now about twice the size of what he used to be- and he was getting bigger with every breath. The growth seemed to pulse in waves, like the tide- He would stop growing for a couple seconds on exhale, and then once he inhaled he would grow bigger and bigger.
"Ahh- Was i supposed to grow this big..?" Tennis Ball's feet seemed to also be becoming disproportionately large, nearly squishing Golf Ball against one of the cold laboratory walls with his giant, fuzzy foot. He was now about 10x his regular size, taking up a considerable amount of the laboratory floor and becoming too large to fit through the lab entrance. He rubbed his massive, fuzzy feet together worriedly.
"Ah- My calculations might've been a bit off-!" Golf Ball said, worried that he would break the roof of her laboratory.
"W-well change me back!!" Tennis Ball said, trying to stand up, stumbling and crushing the ray gun with his knee. "Oops..."
"TB!!" Golf Ball said angrily. "Ugh... Now we're gonna have to find another way to change you back-"
Tennis Ball blushed as he continued to grow, now at a slower pace, as they talked. At least it seemed to be slowing down, but now Golf Ball looked about the size of an actual Golf Ball.
"G-golf ball! You look really tiny..." Tennis Ball said, now looming over his tiny friend. "I hope I don't accidentally crush you...!" Tennis Ball said, suddenly hitting the roof of the laboratory, which was getting really cramped. "Oof!"
"Ah- Tennis Ball, you're huge! Be careful, you might crush my-" Golf ball was cut off by the sound of Tennis Ball's expanding foot knocking over various test tubes and scientific equipment. "-Lab Station. Sigh... We're gonna have to find a way to get you out of here before you destroy EVERYTHING in my lab! Try standing up, it'll collapse the dirt above and allow you to walk elsewhere while we figure this out!" Golf Ball sighed, stressed, but mostly looking up at the now-giant Tennis Ball in complete awe as a fluttery feeling filled her stomach.
Tennis Ball was seemingly sort of panicking as well- He squirmed around as he tried to stand up, exhaling before he inhaled sharply and attempted to stand up- breaking the roof with ease and shaking the ground as a couple of unaware contestants scattered in fear and suprise. Tennis Ball was able to step out of the crater and flop down on the grass, still growing in waves that were increasingly farther apart, panting as his giant fuzzy feet swayed back and forth and he rolled back as he grew and grew, now reaching past the treetops and into the clouds.
"Ahhh!" Tennis ball shifted, accidentally catching Pen and Eraser, who were sitting on a nearby picnic table, underneath his growing paw. He lifted it up to see the two pressed stuck against his sole.
"Oh! Oops, I didn't mean to!" Tennis Ball smiled sheepishly and lowered his feet to the ground, shaking them both off and allowing them to dizzily get their bearings. Pen actually seemed to be in awe of Tennis Ball's new size, putting a hand on his sole that felt like a tickle from a bug. "Oh- Hi down there..."
Tennis Ball seemed to be finished growing as his head was veiled with a crown of clouds, his massive feet rocking back and forth as all the other contestants gaped at the giant looming Tennis Ball.
Just then, Golf Ball emerged from the crater that used to be her laboratory, clutching the MMR. "Ah- TB!! There you are!!" She shouted. "I think that we can count the growth ray as both a success and as a failure- For multiple reasons. This MMR was the only thing that wasn't destroyed or buried-! I'll figure out a way to change you back- for now you just have to deal with being a giant!" Golf ball said, running up beside Pen, who was still staring at his looming paw's sole. "Pen!! Have no fear!! This is probably reversable, I just was testing out my new invention on TB and- Wait, are you blushing?"
"N-No! Of course not! I just never realized how soft TB is. He's so fuzzy... Especially his feet." Pen said, rubbing his paw.
"Hmmm. He is quite fuzzy." Golf Ball said.
"Are you guys talking about me?" TB said worriedly. "When are you gonna be able to change me back..!!" He said, squirming a little bit as he whined. He effortlessly knocked over another nearby picnic table, which luckily didn't have anyone sitting on it.
"Hopefully never..." Pen said, hugging his paw and nuzzling his face in, which made TB embarrassed.
"Typical," Golf Ball said, rolling her eyes. Tennis Ball looked around, his face flushed red, and nervously tried to stay still as to not knock over any more of his friends. He slowly started to move his foot forewards, into a more comfortable position, trying to signal Pen to move out of the way, but he just seemed to interpret it as Tennis Ball trying to cuddle him with his feet. He seemed to be a little ditzy and somewhat fixated on his foot after he was accidentally squished underneath it. He didn't blame Pen- with his newfound size his feet were also a lot softer, like big pillows. Once Tennis Ball was in a full sitting position with his legs extended all the way and his huge soles towering over the smaller contestants, it was a lot more comfortable and a lot easier not to crush anyone. Pen continued rubbing and cuddling his big fuzzy paw.
Tennis Ball smiled slightly, still blushing, and tried to relax more as he sat. As more contestants came to marvel at his giant-ness and feel his fuzzy feet, he wondered how long it would take Golf Ball to find a way to change him back. Hopefully at least a little while.

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hopefully you like! it's a bit rushed and not my best work, but i still hope you enjoy it :'D
Tennis Baaaall my beloved- <3
There's a couple typos in the source file, although they're fixed in the desc version and not that noticeable.