In my experience ive found a direct link to the weight and hardness of the bag to the impact and power you get from training on it
with a softer bag you can hit it in lots of different ways and you wont be punished if you do it wrong,with a harder heavier bag you can only hit it one way and it has to be right,if its not you can feel it pulling on the part of your body that's not lined up so it forces good form more natural mechanics,the more natural the mechanics the harder you hit.
The best filling ive found is kiln dried sand but you may need to build it up slowly to get used to it. Depending on the quality of your bag you may be able to take a bit of stuffing out of the bag and add sand to the top to increase weight but its a idea to put it in a bag inside the bad or it will drop through to the bottom.and turn to stone(makes it more interesting though
Here's a clip of my favourite example of serious power punching on a hard heavy bag,this example is from Dennis Jones and very street orientated but the basics of power punching are the same,he tends to pitch the punch rather than the traditional boxing a bit like fedor
hope this helps