Dry mixed BP powder components is 'meal' powder' which burns in a very inconsistent manner.
Pyrodex, real BP and smokeless propellants all have very carefully controlled particle size = predictable burn rate.
As a water/alcohol paste, that is forced through a screen for sizing and allowed to dry as Black Powder.
Smokeless and Pyrodex look extruded and cut to me.
It's a surface area/volume issue. Cannon BP is pea-size or bigger for commercial fireworks shells 8-12" in diameter.
Clearly, it has to 'push' rather than simply go 'bang'. Lifting a 12" shell to 1,200' for ignition takes a lot of push.