Do Natrelle and Mentor size differently? Is one better on a smaller frame than the other?

The product catalogs for breast implants are based on several implant dimensions including implant base width. When a board certified plastic surgeon measures you, and listens to your desired results during consultation, they should be offering you suggestions based on size that fit your body measurements and give you the profile (natural vs more implanted look) that you desire.One brand or the other is not necessarily better on a small frame. I personally like to keep the breast base width of the implant slightly less than your actual base width as possible, depending on tissue thickness. There may be some trade off with the base width and the profile you want. In general a moderate plus profile will give you about 1.5cups and a high profile about 2 cups (but will look less natural).I would also discuss over/under the muscle based on how much excess skin you measure in pinch test and your relative breast ptosis because you are petite.

In terms of picking the right implants (size and profile/projection) to achieve your goals, generally speaking, I consider two factors when selecting implants for my patients:
1. Dimensional planning and 2. Gel Implant Sizing system
Dimensional 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.  Hence a
physical exam is really needed to give you any meaningful advice.  
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. Basu - Houston, TX

Ultimately, despite the fact that the implants may have different sizes between the two companies there really isn't much difference in the end if you compare apples to apples. In other words, a high profile 350 mL mentor implant would look pretty close on someone as a 340- 360 mL Allergan high profile implant. You might want to try and find a plastic surgeon with a Vectra 3D imaging platform who could  show you exactly, on your frame, in three dimensions, what each of the size and shape implants would look like. Help that's helpful.