I always wondered the same thing until I arrived in Japan almost 4 years ago.
First the history of the anime. The anime is the animation of the manga, first were the books on television. Currently there are many anime that do not have manga, but there are still manga adaptations to anime. That said, the voluptuous animation of women was born in the manga.
In Japanese culture the woman does not show her attributes, if you detail a woman dressed in a traditional kimonos you will notice that the size of the breasts and buttocks you can not see or measure. So what's left when nothing is shown? The imagination! And manga as any cartoon exaggerates the expressions, body, silhouette of all his characters, even personality. The short one and a half meters will measure only half, the woman who more or less highlights breasts even in kimono will look extremely voluptuous. There are also many manga styles, and there are some where these exaggerations are doubly exaggerated ^^, I do not remember what the style is called.
If in addition to this we add that talking about sex is taboo, and that there were no pornographic magazines when the manga came out, because the hentai or erotic category was born. The hentai sleeves were sold in a dark market being the largest buyers the military who went to war. The techniques of drawing the body of the woman for the hentai began to be noticed in the most traditional manga, up to what we see today. And well, those exaggerations sell, so they put them. There are few anime that are not like that, and it will depend on the cartoonist or mangaka, and his audience.
Nowadays, Japanese women are still modest, although the younger women show their legs to a point that I have come to see the beginning of their buttocks. But, the breasts do not! Not even the shoulders or the chest area. A woman with a neckline that bothers more than one Japanese is traditional. So you have to see them in manga and anime.
Cuz it sells. 😈