The amount of sound produced by an engine is directly related to the volume of exhaust gas produced by the combustion. A good anology is a firecracker. The larger the explosion the more gas that is being produced. Gas (not the fuel). The gas causes the sound by being accelerated beyond the speed of sound. Anything moving faster than sound will create a boom that is proportional to the volume of the thing that is moving. This would be more noticible without a muffler. Here's a small experiment: try clapping your hands, first just hard enough to make a sound. Then clap as hard as you can. Notice the difference in the level of sound?