if you are looking for volume, research bodybuilding - its a specific approach. strength training is also different. what you are likely experiencing is endurance (muscle/cardio/etc) training in addition to the skill bumps.
fat wont hold back muscle gain, but it will hide it. good news is that as you more build muscle, you burn more calories. less mass means better cardio, which builds muscle endurance.
good luck with the boxing! its a fantastic martial art.
yes, they are smaller, aerobic, slow twitch muscle fibers. my endurence comment was more related to delivering oxygen to the muscle, but was sloppy. was waiting for someone to poke me over it :-)
smaller functional muscle fibre size is one of the reasons you will often see very svelte endurance athletes.
you can build both slow twitch and the 2 types of fast twitch muscle fibers simultaneously with well planned workouts, but dont worry too much about this. your body will adapt to whatever you train for, just keep a good mix of everything you have mentioned so far and your body will respond.
never neglect the cardio though. a resonable mix of HIIT cardio will add aerobic capacity without sacrificing muscle mass or size. your high rep. punches with 16oz gloves will take care of shoulder endurance.