Bigger tubing is not always better. First it will change the sound of your exhaust, wider motorcycle exhaust tubing tends to be louder but with a lower tone. If you don’t modify your bank so you can bring more air in, you don’t really get a benefit for adding more air going out. Typically, it is the best practice to stick with the same size tubing your motorcycle came with unless you are planning to are a considerable amount of displacement to your cylinders. Your engine was developed to run with the stock diameter exhaust. It’s usually best to use that diameter exhaust even if you are doing some street modifications to your bike.