Assuming you have two whole fleshy arms, if you have a relatively thin forearm, i would suggest treating the prosthetic forearm as being the same thickness as your own (basically either cover your forearm in a thin hard material or just paint it, then making the surrounding components (like the part where her stump joins the prosthetic, as well as the straps, etc) thicker. You can also avoid making the arm look too bulky by also bulking up the rest of your costume in the right places (shoulders, opposite arm), making the prosthetic look a bit more proportionate.