I'm repeating the same info from a similar thread I just started, not having found this one first.
I've tried several brands (various models of Braza, some off brand ones from ebay, Nu-Bra), and models of silicone pads and "chicken fillets" , most are not large enough to produce a full 40B or 42B cup volume given my current scant A cup chest.
I did my homework using the technique often employed by plastic surgeons pre-op for test driving various implant volumes prior to settling on a desired cup size--the rice in stocking trick. Basically, you knot a nylon stocking, fill with various measured volumes of short grain rice and try out in your desired cup size or cup sizes to determine the effective breast volume that suits you best.
I figured out that I needed about 300 mL of added volume to produce an ample 40B cup volume, about 400-450 mL for a 42B (or 40C). The typical "chicken fillet" or "bump pads" are far less than this volume, usually in the range of about 100 mL or so. I did not like the idea of going with breastforms since I have enough tissue volume that provides a very natural contour with added volume using the rice augmentation method weighted mostly at the bottom of the cup. I was not seeking a very large cup size, rather a demure B. Still in a 40, this is a substantial absolute cup volume. See my earlier post where I discuss the "A Bra That Fits" website the concept of sister sizes--40B is the same cup volume as a 36D.
Finally, the solution I found was the Softleaves X99. Softleaves in a British company that makes very high quality silicone breast enhancement devices as well as breastforms. I highly recommend them on the basis of the high quality of the products, superb email support and very TG friendly.
The X99 enhancer is their largest volume enhancer short of an all out breastform. It's made of very soft, high grade silicone gel encapsulated in PU giving a very natural feel nearly identical to real breast tissue and very comfortable to wear. It seats perfectly in a 40B cup. I am almost unaware I am wearing them. They comes in various sizes. I bought the large size X99 which comes in at just under 300 mL volume and works perfectly for me, provides the shape I want and is invisible even with unpadded lace bras. For example I wear them with Bali lace desires, Bali 3383 or Felina Harlow and they are invisible. They are available directly from the company in the UK, shipped quickly and of reasonable but not cheap price.
Thy make quite an array of enhancement pads and devices of various size and shape, the best selection I've run across thus far. I am not promoting the product but just a very happy customer.