Likely down to two basic reasons:
(1) That's what they were taught in art classes.
(2) That's what is most popular at the moment.
Before this, it was the BESM model ("Big Eyes, Small Mouth") of anime production. Exaggeratedly large eyes are one of the most famous and recognisable elements of anime and manga. Large eyes are ubiquitous in shonen and especially shoujo, which also happen to be the two best known demographics outside of Japan. Chibi style, again common in manga and anime aimed at kids and teens, exaggerates the eye size even further. Also it has to add that older shonen (and some newer ones) avert big anime eyes, instead the eyes tend to be either narrower or realistic.
Before that (and continuing as a mainstay of comic books targetted at adolescent boys) was the voluptuous woman (think of any super-heroine of the 1980s). Her clothing is mainly there to accentuate her figure. Her wardrobe mainstays are bikinis/swimsuits, lingerie, fishnets, stockings, garters, dangerously short skirts, equally short shorts, thigh high boots or stiletto pumps, and low-cut dresses and shirts. Alternatively, she might be covered from head to toe, thus showing no skin at all, but her costume will be tight and stretchy enough to show her bust and the outline of her hips.
Currently, the trend seems to be moving toward women with "Hartman Hips" -- a small bustline, a very narrow waist, narrow shoulders and very wide hips often accompanied by large buttocks. Regardless of the width of the character's waist and hips it's important that her waist look exaggeratedly small relative to her hips. The end result should be an exaggerated pear-shaped figure.
We'll see what happens when women finally take over as the majority of Hollywood directors, producers and writers.
