"How'd you come up with the [bottle's] design?" "It just came to me, from who knows where!"
A woman in comics or cartoons doesn't have ribs like a real actress does. Thus she can easily have a far narrower waist than would be possible with even the strictest of corsets - and usually without even wearing one!
A woman with an Impossible Hourglass Figure has generous curves, with an impossibly narrow waist as an unproblematic part of how she just happens to look. Thus taking the classic "hourglass figure" beauty ideal Up to Eleven. She's always designed to be attractive, even in the cases where she's not Ms. Fanservice or Hello, Nurse!: Monsters with insect-like narrow waists do not count, even if the shape reminds of an hourglass.
For more realistic hourglass figures, see instead Of Corsets Sexy and Of Corset Hurts. Compare Hartman Hips.
Not to mention Boa Hancock, Nico Robin, and Nami, for starters, or for that matter pretty much every single female in the entire universe who is not horrendously obese. One would get the idea that this is one of the author's many fancies.
Similar to the One Piece example we have several female characters from Fairy Tail and Rave Master. Mashima states that he actually doesn't find it especially appealing and he mainly uses the design to please young male fans.
As of 2013-2014, Hiro has lessened this trope, giving the girls larger breasts, but a more curvy figure and a evened out midsection.
Quite a few of the female characters have this type of figure in Code Geass, most notably Kallen and Milly.
Winx Club is notorious for these. All females have impossible waists (yes, even the elderly), and in the case of those of the characters who are supposed to be young and attractive, one finds oneself wondering if their waists could even accommodate both the spine and a single pass of intestine, diaphragm and all those pesky not-liver organs be damned.
Madame Mirage was specifically designed in-universe to have this body type — her real identity is a small, athletic blonde with a body as flat as a board from every angle, so (with the help of hologram technology) she disguises as a tall, "impossibly-endowed brunette" based on her dead sister. It's also hinted that there's at least some degree of wish fulfillment behind the Madame Mirage persona's appearance.
This◊ Stella McCartney dress creates the optical illusion that the wearer has one of these.
Fashion from Victorian Britain seems to have been striving towards this at times, with all the corsets and padding and whatnot.
Barbie is notorious for this. The reason this is under fashion design: the narrow waist is allegedly intended to compensate for the heavy seams in such small clothing. Whether this is actually true or not, around 2000 Mattel chose to switch to a wider waist design.
Four of the muses and Aphrodite, from Hercules, have waists so tiny they could probably wrap one hand around them. Oddly enough, Megara ends up with Hartman Hips, despite being the only female to make use of her womanly charms.
Jessica Rabbit from Who Framed Roger Rabbit is more one of the more famous and extreme cases of this trope. She was originally designed with more (relatively speaking) realistic proportions◊, but the creators decided that they wanted an unrealistic quality about her sexiness. She's a toon, after all.
The cosplayer Yaya Han made an excellent Jessica Rabbit outfit with a steel-boned corset, two push-up bras, and a bustle, and she still couldn't match Jessica Rabbit's canonical measurements.
Live Action Television
An episode of It's Always Sunny in Philadelphia actually deconstructs this trope. Dennis takes up fashion designing, and specifically designs his clothes for someone with an Hourglass figure. Of course, since no one in real life actually has one...
Both Ringford Queen Elfaria and Queen Odette from Odin Sphere have pencil thin waists sandwiched in between huge breasts and wide hips. In Queen Odette's case it makes sense considering aside from the breasts she is literately just skin and bone.
Betilla and the rest of the nymph sisters of Rayman Origins surely fit this trope. Fortunately they've got wings, otherwise they'd probably snap right in half.
Almost everyone, in The World Ends with You has a pretty thin physique. Yes, even the some of the boys, including the main protagonist Neku.
It only becomes even more noticeable when the main cast appears in Kingdom Hearts 3D [Dream Drop Distance]. Though still heavily stylized, Sora and Riku's proportions aren't nearly as extreme, making Neku and some of his friends look downright anorexic. It's arguably even weirder considering the characters for both games were designed by the the same guy.
Aeon from Skullgirls takes this trope to extreme levels where her midsection is literally a hourglass.
Barbie in Barbie Super Model has a perfect, impossible figure. Well, it is a Barbie game, after all.
The weapons select screen for Mass Effect 3 reveal that quarian women don't have Hartman Hips, but have this instead; Tali's hips are not significantly wider than Fem!Shep's or any other female squadmate's hips, but her hourglass figure is much narrower.
The Sorceress from Dragon's Crown is the shapeliest of the three female playable characters.
When Candace from Phineas and Ferb is shown as an adult, her figure has gone from literally pencil-straight to improbable hourglass. The mother of three has Hartman Hips naturally enough, but unnaturally has a thinner waist than her much lighter teenage self.
Vanessa already has one, and she's the same age as Candace.