Twilight rolled up the letter and tossed it to the side. She stared at the vial of liquid included in the correspondance.

"What's that?" Spike asked.

"Princess Celestia said it's a variation on a special potion. She wants me to bring it to Zecora to test."

"I don't get it, they have a lab to test at the castle, why doesn't she get the ponies there to do it?"

"I don't know. She said something about an accident that ended up causing some structural damage and flooding. Could be a week or so before its cleared." Twilight reasoned.

"Huh. Sounds like a busted pipe or something. Oh well, it'll give you a reason to get out if the library."

"Yup!" Twilight placed the vial in her saddlebags and put them on her back. "I'll be back later ok Spike?"

The baby dragon saluted as Twilight walked to the door.

"Don't worry, I'll keep everything running smoothly."

Twilight smiled and closed the door behind her. Her mind wandered to her schedule and how this side track would affect it. She remembered she had a book to finish so she could write a report on and pulled it from her saddlebags.

"Never hurts to be prepared."

She walked through town half reading the book and half walking until she got to one part in particular about variations on spellcasting. She read and re-read the paragraph trying to burn it into her brain when she felt something crash into her.

Twilight fell on her side hitting the ground floor and dropping her book.

"Dag-nabbit Twi, I told ya before not to walk around with yer head in them books."

Applejack leaned down and offered the unicorn a hoof which she gratefully took.

"Sorry AJ. I was just reading this fascinating chapter about..." Twilight trailed off

"About?" Applejack asked.

Suddenly the unicorn pulled off her saddlebags and rummaged through them and pulled out pieces of the broken vial still wet.

"Oh no! The princess' potion! I'm so doomed!" The unicorn exclaimed in a panic.

"Easy Twilight. Now whats goin on?"

Twilight's breath stopped. She felt a curious feeling in her lower abdomen. A mix of pleasure and pressure made itself known causing the unicorn to sit down and take stock of herself.

"What in Equestria?!"

"Twilight, whats going on? Ya act like nightmare moon ha- oh my gawd!"

Both ponies were dumbstruck by the small waterballoon-like protrusions on Twilight's crotch.

"Landsakes Twi, yer body is ah acting as if yer heavy with foal!"

Twilight wanted to answer, but that feeling of pleasure returned interrupting her train of thought. The protrusions on her crotch welled up again, fattening up further with milk as her nipples swelled.

"Now ain't that peculiar. I ain't seen anythin like it."

"I... I don't know whats going on Applejaahaaaaa!"

Twilight arched her back. This time a bit of fluid dripped from her sex as both of her crotchtits swelled larger with more milk. Her nipples were already leaking onto the ground forming a small puddle beneath her.

"Applejack... I... Need help... Get Zecora!"

"Yeah, i reckon ya do alright." The orange pony stared as Twilight gasped again. The mare's back hit the ground and her crotchboobs swelled again, this time surpassing the size of a party balloon. Her nipples twitched and fired a stream of milk right at the earth pony.

"Oh hayseed! Careful where ya point those things Twi, I'm just an innocent bystander he--"

Applejack gasped as an unfamiliar feeling hit her. She reached down to where it originated and saw bulges forming beneath her teats.

"Uh oh, i think its happenin' to me too!"

Applejack gasped as she got her answer. The small mounds resting in her crotch filled outward with milk. They weren't anywhere near Twilight's size, but she could tell it wouldn't be long.

The unicorn made immobile by her growing crotchboobs looked around for help. She saw it in the form of Fluttershy flying to the market.

"Fluttershy! Over here-ahhhhhhhhhhhh!" She tried calling out. The twin balloons between her legs surged again bloating to the size of beach balls as they sprayed milk in streams.

The yellow pegasus touched down near them. "Oh my goodness! What happened to you two?!"

"We were jus walking and... Some sort of poti- aaaaaaaahhhhhh!!!!!"

Applejack screamed as a fair bit of juice oozed out of her marehood. The ballooning tits between her legs filled out to the size of balloons and the nipples resting upon them started dripping almost continuously in a stream.

"Here let me help you two up." Fluttershy suggested.

Twilght sat up, no donnnnnnnt! She tried to breath out during another intense orgasm.

Fluttershy walked towards them and stepped into the puddle of milk forming between them. The pegasus stopped in her tracks.

"Oh! Oh my! Wha, what's happening?"

Fluttershy barely finished her sentence as both ponies now enormous crotchboobs sprayed her with their milk.

The effect was immediate and intense as Fluttershy's tiny teats pushed outwards rapidly thanks to the billowing crotchboobs they were attached to. They swelled straight to the size of a party balloon before their eyes.

"Oh, oh, oh boy, it looks like they swell faster, ooooh, when you come into contact with more milk." Twilight reasoned.
"You, you have to get to Ze, Zecora's and tell her whats happening!"

Another orgasm ripped through Twilights senses as she felt her tits bloating again. They were taller than a pony now and her nipples flowed like a garden hose. She was barely able to register Fluttershy flying away trough her addled senses.

"D, don't worry Apple... Applejack. Fluttershy will get help."
Twilight reassured her.

"I sure hope so!" Applejack said as her boobs quaked and jiggled as they started filling again, this time to the size of a beach ball.

Both ponies heard a familiar squeal and what sounded like a crash.

"Twilight craned her neck to see Fluttershy crashed into three other ponies. Her tits had already swollen to the size of balloons and were leaking onto the three other ponies she fell on.

"So... So much for, for that." twilight gasped out. Her boobs had now swollen beyond the size of weather balloons. They pulsed and throbbed as they sent gush after gush of milk pouring onto the ground. She followed the stream of milk as it flowed down the street. An unlucky pony whose tits were now at the Party balloon size and rapidly swelling sat in the middle of the stream gasping between pleasured moans and pounding her hoof on the ground. Further down the stream she saw the milk pouring into sugarcube corner. A massive set of pink crotchboobs were pressed against the glass from the inside.
She looked back at the crash Fluttershy caused and saw four ponies nursing beachball sized tits.

Twilight, out of ideas and feeling her crotchboobs powerfully surge forward once again, charged a spell.

~

"And just what am I supposed to wear to the ball tonight dear sister? My entire wardrobe was contaminated by your little stunt." Luna ranted.

"Maybe you shouldn't wear anything at all. We are princesses after all, who is going to complain?" Celestia added.

"And my quarters!" Luna continued. "Now there is a gaping hole between our rooms! I can't even get any privacy until this is fixed!"

"You mean you can't clop don't you?" Celestia sneered.

A blinding flash of light filled the room followed by the sound of rushing liquid.

Princess Celestia's jaw dropped. Luna and her chambers were filled with gasping mares with absurdly huge crotchboobs growing ever larger. Milk ran in a flood from all of the insanely large mammaries and flowed through the crack beneath the bed chambers doors and down the steps eliciting gasps of surprise and pleasure from the workers below. Everywhere she looked mares with enormous tits filled the space, not the least of which was her faithful student whose ballooned tits were starting to pancake against the ceiling.

"Princess Celestia! Heeeeeeelp!"

"Not again!" Luna wailed, looking down at her rapidly filling crotchboobs.

Celestia ran her hoof across her own crotch feeling her nipples harden and the flesh beneath billow outwards.

"You girls are going to have a hard time topping a gift like this on my birthday." She said licking her lips.

Author's Note

"Oh! let's see Twilight get bigger, let's see Celestia have a go, why isn't Luna next?"

Hopefully this has shattered your expectations. I like the idea of a domino effect and just imagine how the rest of the castle would look right now with a flood of this "special milk" pouring through it.

And by rest of the castle, I mean every mare in the castle.