There were two little stories that went with this. Both concerning Luna and her dealing with her much bustier sister and niece. Done for the Thicc/Bimbo thread on /mlp/
 
“Principal Celestia?”
 
The head administrator of Canterlot High turned to face the student that called her name and came face to face with Sweetie Belle, Scootaloo, and Apple Bloom. Or rather, she came face to bust with the trio since she was so much taller than the freshmen. She leaned down a little to talk to them more directly and almost smiled when she caught the girls glancing at her cleavage. Not that she blamed them—she had quite a lot to look at!
 
“What can I help you with, girls?”
 
They looked up sharply and then got a bashful look on their faces. Celestia sighed knowingly. It must be something of a sensitive nature.
 
“It’s alright girls. We’re alone here. You can talk to me about anything.”
 
Apple Bloom relaxed a little and a gestured towards her friends.
 
“Well, we wus wonderin’…”
 
“You ‘were’ wondering,” the administrator automatically corrected.
 
Apple Bloom blushed. “Right. We were wondering…”
 
Another pause. Celestia gestured for the farm girl to continue.
 
“Well,” Apple Bloom took a deep breath,“We heard that you used to have a pretty small chest and you were that way for quite a while but then, somehow, you managed to figure out how to make your breasts grow and that now you’ll help other girls grow their chests if they really need it and, like, we’re just not developin’ like we ought to and there are girls in our class that are already wearing real bras and can you help us?”
 
Celestia stood there, stunned.
 
Not only had these girls asked her something ludicrous but they somehow knew about one of her most shameful secrets growing up. As a teenager, Celestia had been mortified when her little sister Luna started developing while she was still flat as a board. She had taken to stuffing her bra simply so she wouldn’t look like a boy.
 
Then, she experienced a miracle. As a freshman in college she started taking birth control and her body responded in a surprising way.
 
During that first year away from home her boobs swelled from mosquito bites to H-cups. When she returned home for summer Luna was beside herself with jealousy. That had been the start of a rift between the two that had lasted longer than she cared to remember.
 
The flustered Principal asked how her students knew about her past.
 
“Oh, well we heard the rumor.” Scootaloo said, as she pulled a book from her backpack, “And then we did some research! See?”
 
Celestia realized with horror that the volume was a yearbook from her own senior year. Scootaloo flipped to a page and held it up for inspection. Sure enough, there was a picture of Celestia with her small bosom.
 
“So can you help us?” Sweetie Belle pleaded.
 
Principal Celestia sighed. She hated giving bad news.
 
“Girls, this may not be easy to hear but…”
 
Later that afternoon, as yet another disappointed female student left the principal’s office Luna knocked on the door and entered.
 
“You’ve been very popular today.”
 
Celestia groaned. She REALLY didn’t want Luna to know what was going on but she saw no way to avoid it.
 
That afternoon at least nine other girls, mostly freshman, came to her hoping for a miracle of their own. She gave Luna a look of pure misery and then sighed.
 
“Close the door. You’re not going to believe this.”
 
A few minutes later the school secretary jumped when she heard the vice-principal roar with laughter from behind closed doors.
 
Later, Luna sat in her office smiling to herself. It served her big sister right, what with the way she always wore those ridiculous outfits, showing off as much of her chest as humanly possible! Maybe now she’d be a little more modest.
 
Or not. Luna didn’t really care. She had gotten over her jealousy years ago.
 
What did please her was that little rumor she had started. The trick, she had learned, was to pretend that you had heard it from someone else and then act like you didn’t believe it yourself.
 
And if a student just happened to overhear?
 
Well, it wasn’t her fault that some people were just plain gullible.
 
 

 
 
Cadance had always been close with her two aunts, Luna and Celestia, but more especially with Luna. As the younger of the two sisters, Luna was closer in age to Cadance.
 
Besides that, by the time Cadance was an adolescent Celestia had already gone to college and developed her outrageous figure. The fact that both Luna and Cadance had similar, more modest builds was something else that they had in common.
 
Luna could commiserate with her niece about Celestia. They would even joke good-naturedly about their voluptuous relative. Like the time Luna found one of Celestia’s oversized bras and Cadance quipped that they could use it to go sky-diving.
 
That’s why it had been such a blow to Luna when her beloved niece busted out.
 
She walked into the kitchen one day and heard Celestia talking on the phone. It was obvious by her tone that she was talking to Cadance, but what she was saying was troubling.
 
“Now don’t you worry. I know a thing or two about bra expenses. I’ll bring you some of my old ones. Goodness knows if you’re like me you’ll outgrow them in no time. But at least you’ll have some support! OK. See you soon.”
 
When Luna asked what was going on, Celestia blithely explained that their dear niece had started on birth control recently. Luna felt ice form in the pit of her stomach.
 
She went on that Cadance’s breasts had started getting tender and swollen. Now none of her bras fit and Celestia was headed over with some of her old undergarments so their niece could have something to wear.
 
Luna was numb. Cadance had always been her ally against the big-boobed onslaught of her older sister. Now she was going to join the enemy?
 
It was a silly thought, worthy of a child. And yet…
 
Over the next few month Candace filled out rapidly. Her bosom expanded until it was nearly the equal of her oldest cousin’s.
 
Damn Shining Armor and his need to practice safe-sex! Luna thought.
 
To Luna’s annoyance Cadance and Celestia started going clothes shopping together. No point going with her, Luna thought bitterly, their sizes were so different they couldn’t even buy bras from the same stores!
 
The whole situation, more that anything, was what set her thinking dark thoughts about Celestia. If only Celestia hadn’t broken up their little trio.
 
It wouldn’t be long before Luna and Celestia’s infamous falling out. Luna wallowing in self-pity on one side and Celestia acting just a little too smug and superior on the other. When Luna went to college they parted on very bad terms indeed.
 
It was several more years before they got over their spat and it was Cadance who made it happen. When she announced that she was marrying Shining Armor she asked, no she demanded, that her beloved aunts be her maids of honor. She took advantage of her bridezilla status and forced them to apologize and make up.
 
And, oddly enough, it worked. Celestia had grown up enough to realize that she had been kind of a jerk and Luna finally put her hurt behind her. Being around Cadance helped with that. The soon-to-be bride had adopted Celestia’s habits when it come to clothes but other than that she was still the same old Cadance.
 
One day, when it just the two of them, Cadance surprised Luna when she told her she had taken up a new extreme sport. When Luna asked which one, she watched in puzzlement as her niece reached inside her shirt and wiggled around. Then she pulled her enormous bra out from under her shirt and held it up above her head.
 
“Sky-diving!” she grinned.
 
It had been many years since Luna laughed so hard.