i don't think hitting a heavy bag will do anything for muscle growth, since it is a lot more to do with speed and power training than resistance (which is what is needed for growth). It is also going to act as cardio training, which burns up calories instead of using them later for muscle repair and growth.
Just look at many lightweight boxers - they can hit much harder than a regular person even though they're skinnier than many, and that's because hitting bags will improve your punch, not necessarily your size.
On the other hand, boxing will help to tone up your muscles and maybe give them nicer definition.
You should look to start a weight training program if you wish to bulk up.