I love using a boxing bag. Before I got serious about hitting the gym and putting on size, I used my bag up to two times per day for 30-60 minutes each session. I was very strong even without lifting any weights at all, and had very defined arms, traps, back and core. With that said, I would not consider any of those muscles as being anything close to 'big' at the time.
When I began weight training to gain size, I read a lot about the impact of cardio when trying to put on size. Because of this, I stopped using my bag altogether.
I have noticed that since then (about 5-6 months ago) my stamina and overall fitness has degraded severely. My strength and size however have of course skyrocketed.
I want to start using a bag again, but I am concerned about reducing my gains. I am not sure whether the boxing bag will restrict my gains to the same degree as say cycling or running, but I am much more interested in size gains than my fitness at this stage. I have compared running to boxing and to me it seems like if anything it may even assist with size gains, because of all the muscle groups used. I do still consider boxing as cardio though, which is why I've stopped doing it.
How will my gains be affected if I start using a boxing bag frequently? Will how I use the bag impact on the results? For example, can I hit much harder and less frequently to reduce the impact on my gains vs a more agile and fast-paced heavy bag workout?