Rainbow wakes up one day on her cloud and something just felt different.

"Huh, something just feels odd today. I feel like my body feels... lighter somehow. What's going on?" Rainbow says.

When she gets up and inspects herself and her surroundings, the clouds all around her are enormous.

"What the... why are the clouds so huge all around me?!" she exclaims.

Concerned, she flies to her home and as she approaches her cloud-house, what used to be normal-sized to her are now super-structures. The entrance of her door is now this giant monolithic entrance for giants. But as she enters her home, it all makes sense to her: she has become tiny.

Rainbow gulps, "Oh no, what is this?! I've become tiny or something?! I-I gotta meet my friends in Ponyville like right now! They can't see me like this. Not like this!"

Rainbow attempts to head down to Ponyville with her diminutive size, but is unsuccessful because the winds have become too intense now. What seemed like an easy task for her in her daily life has become impossible.

"Eeeeuhh! The winds are too strong. I can't even get through these clouds anymore. I hope my friends -"

Rainbow's thought is interrupted when she feels rumbling coming towards her.

"W-what is this rumbling?!"

Suddenly, she sees giant yellow and purple-furred ponies coming right toward her at great speed. Rainbow gets disoriented when these bodies twirl her around with the intense wind force they produce.

"W-w-whoa!!"

"Rainbow? Where are you?" a loud voice calls out her name.

"Rainbow, are you here?" a slightly shallow and shy voice also calls out.

After regaining her conscience from the twirling, she looks out and sees a giant Fluttershy and Twilight trying to search for her.

"I'm right here! I'm right here you guys!" Rainbow shouts.

The giants ears twitch and look to the direction of the squeaky voice.

"Oh my gosh, there she is Fluttershy!" Twilight says.

Both ponies whoosh towards tiny Rainbow and stop to contemplate what they're seeing right now.

"Oh my god, she's so tiny!" Fluttershy giggles.

"Rainbow, you're... small," Twilight says.

"Yea, yea I'm small. It was probably that strange candy I ate yesterday. Twi you gotta do something!" Rainbow says.

"Well, I guess you're not the fastest pony anymore, hehe," Twilight taunts.

"CUT IT OUT, TWI!" she shouts.

"C-can I keep her," Fluttershy says.

Rainbow snaps and looks back at Fluttershy just to be a bit intimidated by her size.

"Fluttershy!" she says as she flies a bit back from her giant face.

Twilight shrugs and says, "Hehe, maybe you can Fluttershy. She'll need somepony of our size to take care of her from now on. And I don't know what you ate, Rainbow, but I can't reverse that. I guess you should've read the nutrition label warning for 'shrinking'", she laughs.

Rainbow's size was never restored and she remains a pet for her friends to enjoy. When her friends are relaxing together, they task Rainbow to bring small fruits such as pieces of grapes to each of their mouths. Because of how powerful her friends have become after shrinking down to a miniature size, she's grateful to be still part of their group.

Made in GIMP.

Enjoy!

Sources:

BG:
https://www.deviantart.com/kmanalli/art/Clouds-Background-543096317

Rainbow:
https://www.deviantart.com/orbitalxd/art/Annoyed-Rainbow-Dash-841413492

Fluttershy:
https://www.deviantart.com/kestrelelk/art/You-all-look-really-busy-332531385

Twilight:
https://www.deviantart.com/sketchmcreations/art/Vector-Smugsmart-939329913