Report Abuse. To increase the size or bulk of your muscles you need to do resistance work. That is usually lifting weights or doing body weight exercise. While boxing tones and defines your muscles it is not going to give you a gain in the size of the muscles.f you're doing boxing training to increase muscle and don't plan on competing then yes it'll definitely help you to trim up and build muscle especially if incorporating it with a weight training program. Good luck! Source(s): James · 7 years ago.