the more experience that you have playing the trumpet the more air you have learned to put thu the mouth piece. You will begin to learn to get your air from your diaphragm and not from your lungs which will not let you hold your notes as long. So when you begin you will need a bigger mouth piece so you can get all the air into the trumpet you can get and also as you grow your mouth will to and thus you will need a smaller one so none of the air goes out side or around your piece. It is kind of like a clarinet player and reeds. When you are a beginner you will need a size 2 and then you progress into a different size. If you hold up a sheet of paper and you blow on it and it is a heavy paper you will need more air to make it flutter, as a piece of paper that is thin does not take as much air to make it flutter or move with the wind or air you are blowing on it,the same goes with a mouth piece that you use, the stronger your lips get , and the more you use your diaphragm, the smaller size mouth piece you will need, and the better sound will come out and it will be pure and clear as to muffled and forced, and you can hold your notes out longer.