That is probably pretty much a genetic thing and there is not much you can do about it except build up the rest of your body to make them appear more proportional. I have naturally very large calves and forearms and people often joke that I have "Popeye arms". I don't do any direct forearm exercises, but I don't think it would make any difference. As my upper arms grow my forearms grow with them, so they always look a little out of proportion!