Konata from Lucky Star, though she will only talk about it if someone else broaches the topic.
Konata: A flat chest is a status symbol! Extremely rare and valuable! Tsukasa: I guess I'm all right too.
In the above quote, Konata is referencing the flat-chested Mayumi from the dating-sim game, SHUFFLE! Mayumi is proud of her "lack of endowment", but Rin and his friends like to make fun of her for it.
The same quote later makes its way into Genshiken. Also, Sue was making the comment in regards to Ogiue's chest while the group was at a hot springs, leaving Ogiue a little freaked out between the personal nature of the comment and the fact Sue had been stalking her all day...
Minor character Chichigami from Mahou Sensei Negima! isn't much of an example herself; however, her reaction to one of the main characters being embarrassed of her flat chest is notable.
Chichigami: Large breasts may be fabulous, but tiny breasts, too, are wonderful!
A prime example from Marvel Comics is Jubilee of the X-Men. Anya Corazon, AKA Spider-Girl, also fits this trope.
From DC Comics, Star-Spangled Girl was this for a while.
Incandescence is very comfortable with her smaller bust. It's never mentioned, even though every other woman in the book seems to have a more average or larger chest.
There's a T-shirt that reads: "itty bitty titty committee".
As stated in Aikan Muyo
Aeka: More than a handful is a tongue-sprainer.
The perfect rationale for why many women prefer to have smaller boobs came in Some Like It Hot. Sugar remarked that she admired "Daphne"'s flat-chested appearance: "Clothes hang better on you."
Lampshaded in the movie Thoroughly Modern Millie, where flapper Millie (played by Julie Andrews) complained that her "fronts" made it difficult to wear her beads properly. She tries to flatten her chest but she's just too well-endowed.
In Weird Science, the two protagonists consider designing the program to make the girl's breasts huge; but then Gary says, "Hold it, Wyatt, I know female stats. Anything bigger than a handful and you're looking at a sprained tongue." So they reduce them to average size. (Zigzagged in a way because while Lisa'a bust doesn't turn out to be huge, she isn't exactly petite either.)
In Discount Armageddon by Seanan McGuire, Verity Price expresses relief that the (very literal) boob fairy has been sparing with her gifts to women of her family. Some gunplay by her grandmother, Alice Healy-Price, several decades ago may have had something to do with it.
Mary Russell, the title character of the Mary Russell series. She is not saddened by her boyish figure, and even said that her shape was popular in the current '20s.
Cathy Ryan, Jack Ryan's wife from Tom Clancy's Jack Ryan series, sometimes comments on her lack of a bustline.
Chilli of TLC has gone on record of saying she used to feel self-conscious about the size of her breasts, and it took a while to make peace with it. This was addressed in the video for "Unpretty". She plays a women whose boyfriends suggests implants. However, when she goes in to have the surgery, she sees a (disturbingly young) girl rather graphically having her implants removed, she runs out of the room. She's also jokingly said that TLC stood for "Titty-Less Crew".
Stand Up Comedy
Maria Bamford occasionally draws attention to her (lack of) assets in the chesty department. For example, she notes with more than a little sarcasm that men can't get enough of "these 32 AA's", describes them as "perfect, rare, delicate miniatures". More generally, as part of a larger bit on body issues she noted that she somehow managed to master being thin, but then they start laying the expectations.
"Skinny is in, I've got that covered. But now you have to have muscle tone too! I'm already hungry and tired; now I gotta go work out?!"
Marcel Lucont wrote a comedic poem about how small breasts are so much better than larger breasts, and that women should be proud of them.
Ana Radic, a Serbian comedian, is greatly overjoyed that men pester her more well endowed sister and has described her modestly endowed chest as a gift that allows people to focus on what she's saying. Considering most of her humor comes from being The Girl Next Door and a Shrinking Violet, she has a very strong male fanbase in her home country despite not being as chesty as the Serbian media says a woman should be, and she is thrilled about it.
"It turns out, in comedy, being funny is important! Who knew? The world is crazy like that."
Rita in Tales of Vesperia comments that she's glad she has a flat chest, because boobs the size of Judith's would bounce around too much and make it difficult for her to concentrate.
Suika Ibuki from Touhou is often portrayed as having this in Fanon. In fact, there's an entire song titled Tsurupettan, complete with a video (warning: very addictive!), which is essentially a high-spirited hymn to pettankos all around the world. In case you are interested in the origins of the song and video, refer to this post.
Umineko borrows the song, when Jessica cosplays as Marisa in EP2 of the original sound novel (5th episode of the anime). However, Jessica herself isn't this.
Archers in the Disgaea series are proud of their (lack of) assets, in stark contrast to most other Pettanko characters. Probably because boobs get in the way of shooting properly.
In Disgaea 4: A Promise Unforgotten's DLC Nether Battle tournament, there's a scene where flat characters talk about the difficulties that come with being buxom, and how lucky are they don't have to worry about that. Then it's revealed they've been attacking every buxom character they come across.
Played for laughs in Misfile when resident Pettanko Missi immediately switches from A-Cup Angst to Petite Pride because Ash accidentally gave her the impression that she prefers small-breasted women. Missi instantly goes from angsting over her flat chest (her Imagine Spot is the page image for I Have Boobs, You Must Obey!) to trying to capitalize on it.
Mistress is very flat-chested but given the kind of clothes she wears, it's probably safe to say she's not self-conscious about it.
Greir is also small, but it never seems to affect her.
In Magience, Crystal doesn't like having large breasts, so she deliberately gives Rune a flat chest.
Hydra: Flat as a washboard. You're practically concave! Rune: Yes ma'am!
In the "Beefsquatch" episode of Bob's Burgers, Linda goes on Hey Good Cookin', pulls up her shirt and reveals her 36A breasts on live television.
In previous centuries, small breasts have been viewed as preferable. At different periods, various styles of corsets and stays were designed to flatten the chest as much as support it.
A famous historical example; Marie Antoinette was seen as having comically large breasts in Versailles for a slightly larger than average bust. Small breasts weren't so much in fashion as the only fashion for the french court.
So popular in The Roaring Twenties that there was underwear produced to flatten the chest. The "washboard" look was meant to stick it to the man in the early days of feminism, the same as wearing too much makeup and cussing was. In other words, it was all about projecting an un-ladylike image.
An early bra company called Boyishform specialized in bras that flattened the chest. Maidenform was actually created in response to the "washboard" look and wanted to bring back the curvaceous look.
The bulk of supermodels through The Sixties and The Nineties. The perceived attractiveness of this is apparently a Cyclic Trope. Like Twiggy, Milla Jovovich (until her pregnancy), or Kate Moss. Twiggy was one of the only supermodels of her period to have a boyish figure. Prior to her modelling career (and indeed afterwards, when the 'Perfect Size 6' occurred in the 70s and 80s), models were normally voluptuous. A boyish, flat-chested physique didn't become truly in vogue until the 'heroin chic' look of the 90's.
Director Billy Wilder said of Audrey Hepburn, "This girl, singlehanded, may make bosoms a thing of the past." He was, arguably, not far off target.
Though everything else about Paris Hilton may be fake, she insists that she will never get a boob job because she likes her "tiny chest" and thinks it looks better in the couture designs she wears constantly.
Debra Messing. She was the butt of more than a few flat-chested jokes on Will and Grace, but has said that she doesn't subscribe to that standard of beauty, and is happy with what God gave her.
Keira Knightley. She's been described as "Sunken Chest and No Booty". Specific examples from her films follow.
While her makeup in Pirates of the Caribbean was designed to create the illusion of cleavage, she is proud of her small chest. Captain Jack couldn't resist taking a shot, though:
Sparrow: So unless you've got a rudder and a lot of sails hidden in that bodice— (scans her briefly) — unlikely...
In the commentary for the first Pirates movie, Keira gleefully comments on how impressive the costume/makeup artists made her cleavage look (fully accepting that she does not have boobs in real life.)
Despite being flat as a board, posters for King Arthur were doctored to make her look like a C-cup. Keira Knightley was unhappy with her posters being photoshopped to make her look bigger.
In Domino, a girl in her boarding school makes a snark about her adolescent boy physique, prompting a beatdown.
An example that's very similar in nature to the "King Arthur" movie poster example cited above (very similar and arguably even more stupid), albeit unrelated to Keira Knightley, came with the IMAX release of "Harry Potter and the Order of the Phoenix", the poster for which showing a digitally enhanced version◊ of Emma Watson's still developing figure (she was sixteen at the time the movie was shot and while Ms Watson didn't turn out to be particularly well-endowed as an adult either, she still has developed notably since then).
Former (maybe) porn star Brianna Love, chest-wise might as well have never hit puberty even while being pregnant, but like the above statement puts focus on something else.
After tweeting a picture of Cat's excessive bra wardrobe including the 32A tag, Ariana Grande obtained — and accepted — the nickname Boobiana Petite.
While not because of beauty reasons, there are quite a few women who enjoy having smaller breasts for pragmatic reasons. It's easier to find shirts that fit, you don't have back and shoulder pain, and in general, they enjoy the fact that they can pad themselves up if the situation presents itself, but don't have to deal with it all the time.
While stores tend to stock fewer bras in the smaller cup sizes, they are still easier to find and usually less expensive than bras for women with very large cup sizes.
There are in fact women who feel quite beautiful with small breasts (be it despite them, because of them, or the whole topic just being a total non-issue).