Breast size and proportion in a drawing.
Breast size, how to draw small, medium and large breasts.
As I have already mentioned the female breasts vary so much in shape and breast size that I believe that there is no hard and fast rule on how they should look in your picture. However, it is always best to try to draw natural looking breasts. Unless of course, you are sketching breasts on a Manga or Hentai character in which case the sky's the limit.
Examples of breast sizes.
Below are some examples to help you decide the size and shape of the breast in relation to your subject. Although not always the case the size of a women's breasts usually match her build.
So if you are sketching a petite model then I would suggest giving her smaller breasts as aesthetically they will look far more natural than if you gave her large breasts. That being said it is really entirely up to you and what you see as aesthetically correct.
In this first example, the subject has small breasts. It is important that you position them correctly as more of the upper body can be seen. Therefore it becomes more obvious if the breasts are positioned incorrectly. This is due to the gap between each breast being wider showing more of the chest. The rib cage is also more visible making it easier to make out the overall shape and proportions of the upper body. Also as there is less weight in the breasts making them appear to sit slightly higher on the chest. Of all breasts, I find small breasts harder to draw as often there is little shadow or detail to work with. Also, the nipple tends to be a lot smaller and is harder to define.
In this example showing medium sized breasts, you can see that less of the upper body is visible. The gap between each breast becomes narrower creating the beginnings of cleavage. Also, the rib cage becomes less visible as the breasts are fuller in appearance and take up more of the chest. You can also see that the upper body shape is slightly less visible nearer the armpits. As the breasts get larger they become heavier and more elongated and will appear to hang more on the chest.
In this example showing Big breasts, the gap between each breast disappears and a cleavage becomes clearly visible. The breast is fuller still and covers most of the rib cage and can clearly be seen beyond the outline of the upper body. Being bigger and thus heavier than the medium breasts they sit slightly lower on the chest and the nipples are larger and more pronounced. See also