To explain this difference further, there's some inherent practical limitations on blackpowder which really limits what it's suitable for.
First of all, blackpowder burns quite dirty, it leaves quite a lot of carbon residue with every shot, for a musket this is ok, for a revolver or manual repeater this is acceptable, but for a self-loading action, this is an absolute dead end, it fouls so fast that you'd be lucky to get through a few dozen shots without a malfunction, and assuming that you do, you still inevitably reach the point where it gets so dirty that it just won't cycle by its own power anymore.
Second is that blackpowder is just blackpowder, there really isn't a stronger blackpowder or a weaker blackpowder, it's one kind of substance, if you want more power, you need more volume.
For example, the .45-70 Government was the rifle cartridge of choice for the US military once upon a time, .45 is the caliber, 70 is the number of grains of black powder.
If you wanted to put more power behind a .45 caliber projectile, you would need a larger case to fit a larger volume of blackpowder, thus the .45-90, and the .45-120, for 90gr and 120gr of blackpowder respectively.
Third, there's just an upper limit to how much velocity you can get from blackpowder, even if you use a small and lightweight projectile, adding more and more powder really isn't going to make it go much faster, your returns diminish, thus, to really get good power, you would need a larger and heavier projectile backed with a large enough load of blackpowder to send it flying at a good speed.
This is why military muskets were usually of bore diameters of like .68 caliber, .75 caliber, and even larger, just shooting a lead bullet the size of a pinball, you had one shot and you had to make it count, because reloading is a project, but even with repeating cartridge rifles, people were still going with .40 caliber, .45 caliber, .50 caliber, .57 caliber, etc, because it just had to be big.