Thank you for your question and photos. An examination in person would be required to properly evaluate you and make specific recommendations. That said, based on your photos and dimensions, the 450 is almost certainly going to give you an exaggerated unnatural look which you would like to avoid. You appear to have generous breasts and if you are looking for a rounder fuller look and Moderate (classic) or lower profile implant should suffice. Volume (300 versus 400), is far less significant than implant width and projection. Implant width is similar to shoe size. The implant literally needs to fit into your breast and chest. The 450 implant may well “fit” your breast, but the volume and projection are what will determine the “look” you want, similar to the style or type of shoe you would select. It is true that for a given volume a high profile implant will have a narrower diameter, and it is also true that a person such as you with a petite frame and full breasts should require a wider diameter implant to enhance fullness and roundness. Other considerations such as saline versus silicone are also important, as are "round" versus "anatomical" or "form stable" or "shaped", smooth surface versus textured, and softer versus firmer with the different levels of gel cohesivity.
