Reasons for This Trope
One reason for this trope is that Men Are Generic, Women Are Special, with males being perceived as the default and the females being "othered."
In a few species in the animal kingdom, like many birds, some mammals, and anglerfish, the sex dictates a huge visual disparity, yet it is nearly inevitable that it is the female anthropomorphic character who loses the animalistic traits. When anthropomorphic male animals are illustrated, they're allowed to retain the look of the respective species they happen to be, but when female anthropomorphic characters, the artists are pushing the immediate visual recognition of femininity which results in "prettifying." In some instances, sexualization of the original animal character.
Another reason is that, as we are a species largely used to seeing our fellows clothed, it is only logical that we would come to see masculinity as an absence of feminine traits.
After all, while the physical attributes that most obviously designate one as male are completely rendered veiled by socially normal attire, a woman under normal circumstances would have to put particular effort into disguising the presence of her breasts, the shape of which are generally obvious even when they are concealed. Consider also how many distinct styles of clothing are considered "male", how many are "female", and how many can easily be worn by either side; we may be pinning much of our ability to distinguish between sexes strictly on the presence or absence of a feminine figure.
How Media Tends to Proportion Anthropomorphic Female Animal Characters, and Its Effects
The tendency for media to sexualize women more than men does not apply only to human and Demihuman characters. This tendency also extends to alien, monster, and animal characters (fictional or nonfictional species). With all three, the females are usually made more humanoid than the males as well.
The media tends to depict bipedal or anthropomorphic female animal characters with mostly or completely humanoid body proportions and human-like breasts rather than as a bipedal animal. For reference, a human torso is about three head lengths, legs are another three head lengths, and arms are three-and-a-half head lengths (we'll call this the 3-3-3½ ratio). Cats, dogs, and lizards have a 4-2-2 ratio. They tend to be sexualized as well. An early example of anthropomorphic female animal characters who are on the more "human" end of the sliding scale of anthropomorphism with humanoid body shapes is Van Beuren Studio's feline character in "The Farmerette," done in 1932. There were a few other early examples, but female animals of this level of anthropomorphism have become more common since the 1980s. Due to this, it is now natural for a naked or even partly dressed female animal character to seem awkward to the viewer unless she is a Nearly Normal Animal, Talking Animal or Partially Civilized Animal. This tendency is even greater with fanart of female animal characters in media.
This trope arises from artists who tend to associate "feminine" tells with human female Secondary Sexual Characteristics when drawing female animals. By contrast, the designs of male and prepubescent female animal characters are usually composed of basic shapes that remain faithful to their species rather than having a humanoid body shape because male characters are not held to the same standard. Intimidating, macho, muscular, or evil anthropomorphic male animals may be drawn with a broad chest, a square jaw, or a top-heavy build to show those traits, but you would think less that it was male and more that that was just how the character looked like. Prepubescent female animal characters are not held to the same standard because prepubescent girls aren't really either.
The more animalistic look of male anthropomorphic animal characters and more humanoid look of female ones may reflect society's expectation of females to be more "refined" and the males to be more "savage"/"beastly", in attempts to cultivate greater identification with the characters based upon gender roles.
Even though there are a lot of anthropomorphic female animals that are practically human, there are still some female animals that more resemble their species. For example, Mrs. Brisby (notice the bent hind legs) and Penelope Pussycat (notice the shorter limbs).
Anthropomorphic Female Animal Characters With a Funny Animal/Civilized Animal Body Shape
- Mrs. Brisby from The Secret of NIMH
- Gloria the hippo from Madagascar and Gia the jaguar from the third movie.
- Tigress from Kung Fu Panda doesn't have human-like breasts or Hartman Hips. Instead, she looks lean, sleek, feminine and sexy without abusing feline anatomy.
- Kitty Softpaws from Puss in Boots
- Abby Mallard, Foxy Loxy, and Goosey Loosey from Chicken Little
- Olivia Flaversham from The Great Mouse Detective
- Miss Bianca from The Rescuers
- Sawyer from Cats Don't Dance is rather like the Gadget and Rebecca examples below.
- Lulubelle from the "Bongo" segment of Fun and Fancy Free
- Lady Kluck from Robin Hood
- Kanga from both the original and Disney versions of Winnie-the-Pooh
- Arlene from the Garfield comics, shows, and specials
- Dixie Kong and Wrinkly Kong from the Donkey Kong Country games
- Minnie Mouse and Daisy Duck from the Classic Disney Shorts
- Ortensia the cat and Francine Cottontail from the Oswald the Lucky Rabbit cartoons.
- Penelope Pussycat, Ma Cat, a little-known yellow and white tuxedo cat named Sylvia, and Miss Prissy the hen from Looney Tunes
- Babs Bunny, Fifi La Fume, Sweetie and Shirley the Loon from Tiny Toon Adventures
- Dot Warner, Slappy Squirrel, Marita the hippo, Candie Chipmunk and Rita the cat from Animaniacs
- Marlene the otter from The Penguins of Madagascar
- Heathcliff's girlfriend, Sonja, has the same round body type that he has.
- Cindy Bear from Yogi Bear
- The female animals in Father of the Pride. A few of the female animals may have Hartman Hips and one of the lionesses may have a chest shape that suggests breasts, but they all keep the basic body shape of their species.
- Bessie and Abby in Back at the Barnyard.
- Bear, Sheep, and Kangaroo from WordWorld.
- Gosalyn from Darkwing Duck
- Molly Cunningham from TaleSpin
- Gadget Hackwrench of Chip 'n Dale Rescue Rangers and Rebecca Cunningham of TaleSpin do have somewhat more humanoid proportioning than the other mice and bears respectively, but they are not totally humanoid the way some forms of media proportion female characters.
- Tillie the tiger cub from the Silly Symphony, "Elmer The Elephant''.
- Some of the female cats are no more anthropomorphic than Tom or Butch.
- Any bipedal female character in Dinosaur Train can count as this as they are no more humanoid than the male ones, and they keep the basic body shape of their species.
- Daniel Tiger's mom, Katarina Kittycat, and her mom in Daniel Tiger's Neighborhood
- Hallie the hippo and Lambie the lamb from Doc McStuffins.
- Nicole Watterson the cat, Anais Watterson the rabbit, and Tina Rex from The Amazing World of Gumball.