I Have Small 34B Breasts, Can I Go to 34D Breasts Without Looking Too Fake? (photo)

Thank you for your question. When a patient mentions natural looking, they are normally referring to the upper pole fullness of the breast after breast augmentation. There are two considerations with breast augmentation as far as implant selection goes. Shaped implants and lower profile implants will decrease the amount of upper pole fullness compared to the higher profile round implants. In addition, size will increase the entire breast, so a very large size will fill out the upper portion of the breast to a greater degree than a smaller size.
I would recommend visiting with a board certified plastic surgeon in your area to discuss your concerns in more detail and try some external sizers to get a feel and visual of what you will look like.

Breast implants naturally give more fullness to the upper pole of the breast.  A submuscular placement adds to this fullness.  There is no exact right number of cc’s to use since  every body is different and everyone’s desires and what they see is different.  Typically somewhere between 275 cc and 400 cc is needed to provide adequate shape and fullness, but this will vary depending upon the body type. Be sure to discuss your desires and possible outcomes with your surgeon. Implant sizers and looking at a 3 dimensional image of your body with implants often helps.

To answer your question about cc, you first need to take into account what's known as your base width diameter.
Imagine lying on your back and drawing a circle around your breast. If you were to draw a line across that circle, it will be a given length. Your plastic surgeon will measure this length, which is called ‘’base width diameter’’ or BWD for short. For women, the base width diameter can go from 9.5 cm to 16.5 cm, or 3.75 to 6.5 inches. As you can see, there is a very wide range. 
300 cc on a woman with a BWD of 10 cm is very different than 300 cc for a woman with a BWD of 15 cm. On one it may be enough volume, and for the other it may be much less than the max she can safely go. 
Not only does it determine how big you can safely go, but also how big you can go without looking fake. So you need to have your measurements taken to best answer your question.