Why does 400cc look larger on a woman with little breast tissue than on someone with more tissue to begin with?

Thank you for the good question! You are absolutely correct in that the same volume of breast implant may look very different on different women.  Factors that may come into play are the amount of native breast tissue present, the shape of the chest wall (concave versus convex), the type (saline versus silicone) profiles (moderate versus high  profile for example) of breast implants used,  the distance of the photographer from the subject…
Generally speaking, the best online advice I can give to ladies who are considering  breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as  “natural” or "C or D cup”  etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate. Again, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size.
 The use of computer imaging technology may also be very helpful during this communication process.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this (and the attached link) helps.

There are many different measurements to consider other than height and weight when selecting the ideal size of a breast implant.  If you look at my breast implant before and after gallery, you will see many women with implants of the same cc with very different results. Some of it has to do with the shape if the breast implant, some is related the chest width. Be sure to see a plastic surgeon who uses biodimensional planning techniques so you can be sure you are getting the ideal size for you. 

There are several variable that determine the result from implant sizes.  Generally speaking, I consider two factors when selecting implants for my patients:1. Dimensional planning2. Gel Implant Sizing systemDimensional planning – The measurements of your chest wall are taken. Also, the breast dimensions including the height, width, and current dimensions of each breast form the basis of dimensional planning. Based on these measurements, the implant size is recommended. This will give you a unique breast implant that is suited for your body frame. However, there are some limitations of what size we can recommend. For instance, some implants may just be too big for a narrow chest wall. Your surgeon can review this with you during the consultation.Gel Implant Sizing system – During the preliminary breast implant consultation, you will be provided with an option to “try on” a variety of implant shapes and sizes. You can also visualize the possible outcomes of your surgery which helps you to get that perfect size to give you the shape that you longed for. This way your preferences are known and you can then pick a range of implants that will “fit” just right to give a soft natural fuller look. Hope this helps.Dr. BasuHouston, TX