A two stroke needs to have back pressure to stop the outflow of gas from the cylinder the very moment the transfer ports close. If the outflow is not stopped in time, fresh mixture escapes and there is less to burn. You loose power that way.
Without getting into complicated stuff, the stinger (outlet) can have about the same size as the carb. The exhaust port is much bigger than that, but that is only needed to get all that gas pressure down before the transfer ports start to open. The much needed back pressure can be adjusted with the stinger diameter and first expansion volume of the muffler system. There seems to be an inverse ratio between chamber volume and stinger diameter. Small volume chambers only perform very well if the stinger diameter is large. Large volume systems can have much smaller stingers, and thus can achieve better noise attenuation without power loss.