Assumptions: Though impossible for ponies to have breast sizes, the equivalent occurs where Rainbow Dash is rather sensitive about her "lack of curves." Along with her tomcoltish nature, this leads to very unflatteringassumptions.
Generally speaking, it's an enormously popular trend in Friendship is Magic humanized fanworks to portray Rainbow Dash as the most petite member of the main group. Depending on the writer/artist, she's usually proud of her small breasts (being an athlete)... but just as often you have her secretly overcompensating to hide her insecurities when comparing herself to Fluttershy, her best friend since childhood and popularly depicted as being the most buxom of the six.
Austria's gender-bend is portrayed this way in fanon. Austria and Liechtenstein are seen mourning about it in a webcomic on Pixiv. Also, one artist also on Pixiv drew her, Prussia, and Germany: with Germany at the big size (G Cup), Prussia at the middle size (E cup), and Austria as the small size (B cup). This is because Word of God said he applied A-Cup Angst to Liechtenstein because he saw that women in Switzerland and Austria ranked lower in breast size.
Japan's gender-bend also gets this treatment, especially since the canon sketches of her don't exactly emphasize her assets the way sketches of many other Nyotalia nations do. In one Pixiv fanart◊, she's shown weeping about having no chest compared to the extremely well-endowed Greece's.
Date A Re:Live: Mana Takamiya is very annoyed by the small size of her chest (78 cm), and every time a Spirit shows up, she overreacts to how noticeable larger they are compared to her. Especially once Kotori Itsuka's body gets an Overnight Age-Up from how her Inversion happened (and Kotoris the got the smallest breasts of the Sephira Spirits), and the revelation she's Half-Spirit yet her Spirit half hasn't amended this issue, especially when her mom( Rinne Sonogami) mentions she might never grow any bigger. No longer a worry as of the Shido Countdown arc, where she wakes up on her birthday to discover she had a huge growth spurt that's caused her breasts to become 91 cm, her joy at the revelation is clear.
During Chapter 2 of Despair's Last Resort, Shizuka Matsuki tries to claim she doesn't have this. She fails to be convincing.
Evangelion 303: Inverted. In chapter 15, Asuka teases Kelly about her -Kelly's- chest being too big to play rugby properly.
Rin Tohsaka gets annoyed by certain characters' large chests, especially her own Servant Archer/Francis Drake. She gets really pissed off when Drake brags that she already had big boobs at age 13.
Illya is upset because she is already 18 but flat chested since her body's growth was halted. She has a mini rant that this is probably the reason why Shirou hasn't noticed her.
"Girls Exclusive": This Hunter × Huntersmut fanfic details Gon and Killua being turned into girls by a rainbow-haired girl's Nen ability. As girls, Gon is gifted with a busty appearance while Killua is flat-chested, at first he isn't really bothered by it, but upon Gon commenting on the difference in their sizes and some experimentation, Killua gets pretty jealous and laments his small size, to even his own surprise.
In Heart and Soul Tracey Davis is ecstatic over her new developments summer after fourth year.
Tracey: Do you have any idea idea how long I've been waiting for these? I was afraid I'd never get them! And then this last term was the absolute worst because we spent so much time hanging around Susan - she's so lucky, I totally hate her.
Pokémon Reset Bloodlines: Implied for Lana. Her first reaction to meeting a girl named Iolani, who is rather talland curvaceous is uttering the words "I am envious". A later sidestory has her telling her classmate Velvet Lono, who has a similar frame, that her breasts are "large and enviable".
Tales of Faith: Cress falls under this. She would usually beat up any guy in a fit of rage should she hear them state about her chest in earshot. When it comes her comparing her chest to other females, however, Cress becomes greatly embarrassed about her small size, despite being one of the eldest females in the party.
In Total Drama fanfic series Unbreakable Red Silken Thread While far from flat chested, Heather is somewhat self-conscious about her boob size. Her friendships with Lindsay and Jasmine don't exactly help with this, though it becomes even more apparent after Gwen shows up with some rather generous "development" since they last spoke.
In the Splatoon fic I've Got Your Back, Pearl doesn't like her body. Her chest and back are her two main points of disgust. Her chest is barely developed and she hates it.
In the somewhat NSFW X-Men fanfic "What Happens In Vegas", a few days after Kitty Pryde, Piotr Rasputin and Rachel Summers have a drunken threesome, Kitty admits to her own insecurities regarding her figure. She laments to Piotr that Rachel's body would be at home on the cover of Maxim alongside Evangelista and Mary Jane Watson, while Kitty herself has what she describes as a dancer's body; "In other words, I look like two tangerines on an ironing board."
Xenophilia features another variant of this trope with, again, Dash. When she and her human lover are discussing her previous bad luck with males, she mentions a stallion who used her for a one-night stand and then left saying he wanted a mare with "more jiggle in her rump". Pettanko mares have flat rumps but the result is the same, with all the angst and feeling ugly that would imply.
Ghost: Let's get this started before we get any more distracted! Hyuuga Hinata a.k.a. the only Konoha kunoichi that has hit puberty so far... Sakura and Ino:[look at Hinata then themselves and proceed to sulk in the corner] I'm still pretty.
Kantai Collection fanworks really like applying this trope to the less-endowed girls:
This trope is so ubiquitous that when Ryuujou, an aircraft carrier that actually prides herself on her "distinctive silhouette" entered the fray, fanon naturally assumed she was lying and subjected her to all sorts of breast envy and flat jokes. Pixiv calls the practice "RJ Bullying", and Danbooru has a dedicated pool for collecting some of the crueler jokes off the internet.
Some of the other flat-chested non-destroyer shipgirls (Zuihou, Abukuma, and Taiyou to name a few) get breast envy jokes thrown their way, though never to the same extent as Ryuujou. In fact, a given joke might have Ryuujou try cheering up the target by being as proud of her chest as she is in canon, only for the girl to take it as an insult and consider her lax attitude about it as "giving up" on their figure.
Many Kim Possible fanfics portray Kim as being self-conscious that she's flat-chested.
Fanon for The Loud House is that either Luna or Lynn have this. Lynn usually just because she's only 13 and still growing but that doesn't prevent her worrying that she's going to be flat and therefore unattractive forever. Luna meanwhile will usually feel it fairly severely, given that she's 15 and her two older sisters (by fairly small margins) are notably bustier.
There are quite a few Pretty Cure fan art that just love to have most of the girls give hell to some of the others, with Love Momozono/Cure Peach of Fresh Pretty Cure! and Hibiki Hojo/Cure Melody of Suite Pretty Cure ♪ getting the worst of it for the leader-types. This is due to the fact that, thanks to various artist designs for the characters, the Fresh and Suite teams do have visible breasts while most everyone else is drawn completely flat.
Fanfiction for Ranma ½ goes a lot further in milking Akane's canonical bosom-related insecurities than the show ever does, with Akane both openly lamenting her small-by-comparison breasts and being actively teased about her "flatness" by Shampoo, female Ranma, and occasionally Ukyo.
Weiss in RWBY fanfiction is often depicted with this mindset in relation to those around her such as Yang and Pyrrha.
Touhou fanart, in general, splits the cast into either flat or gigantic... and tends to represent one of each in fanart:
Parsee (a youkai that feeds on jealousy) is not only flat but hangs out with the oni Yuugi, the biggest (huhu) example of Boobs of Steel in the series. Suika (a Petite Pride oni) occasionally gets the same.
Sakuya is forever disciplining Meiling for slacking off, with many comics adding jealousy as a motive (another running gag is that she wears padding that escapes at the least opportune times).
Similarly, Eiki the judge of the dead is forever telling off Komachi for both slacking and having big breasts, which contributes to her Hair-Trigger Temper.
The "baka quartet" of childlike characters are usually seen annoying Yuuka, yet another Boobs of Steel contender.