Remember, wood which is twice as wide, is twice as strong, but wood which is twice as thick, is eight times as strong. In other words, a small change in thickness is significant. All else being equal, the difference in limb thickness between a 70# bow, and a 35# bow is not much. Wood is an organic material, and there are no real hard and fast thickness dimensions, like when you're assembling a FG bow. You have to remove material, teach the limb to bend, and arrive at your intended draw weight and length, all the while keeping the limbs bending together and evenly.