There are several factors in play. The first one is black powder is measured by volume whereas smokeless is measured by weight. There is a huge difference between the two.
And you would be 100% wrong.
We have many threads and posts on this but to recap- many bp shooters do measure by volume BUT many measure by weight+volume (competition shooters). Smokeless reloading (and yes, I've been doing it for 40+ years) is done the exact same way. Many bp shooters are pretty cavalier in their attitude to powder measurement and just use volumetric means and that's because bp is quite forgiving in that sense. BUT if you want accuracy, you'll get just as OCD about measuring (meaning weighing) as any smokeless reloader, if not more so.
So in measuring a charge of smokeless (or bp for the competition shooter), the reloader will set his powder measure to dispense a certain weight of powder BUT, that very powder measure works on
VOLUME! The only exception to this is the new computer driven "trickler" type of measure that will dispense (again by volume) a certain amount of powder onto a scale and the computer will "trickle" powder on the scale till the desired charge is met. So it's a marriage of volume and weight to precisely measure a charge. That's a level of OCD many smokeless reloaders don't go to unless they're competition shooting where accuracy is king. Back in the day before the computer controlled "tricklers", in loading precision ammo for my rifles, I'd dispense a charge from my powder measure that was a bit under what I wanted. I then put that charge on a scale and "trickled" powder till it came up to the weight I wanted. Same thing, older tech.
Once again, you cannot violate the laws of physics. Mass is Mass, whether black or smokeless powder. A given mass of a given composition will have unique burn characteristics, and that holds for black or smokeless powder. Where things get sticky with regards to smokeless is the composition can yield wildly varying burn rates and peak pressures while black powder is somewhat limited in that regard. Smokeless burn rates and peak pressure are varied by
chemical composition (which varies widely) AND size of individual powder granules while Black Powder is pretty much limited to the ancient ratios of sulphur, potassium nitrate and charcoal coupled with the size the granules.
Now to the use of smokeless in traditional muzzleloaders- a VERY bad idea. The traditional guns were never engineered for smokeless, although it is true that there are some modern muzzleloaders designed specifically for it and there is another place to have those discussions. Using smokeless in an original gun with it's steel from the "day" maybe of suspect quality compared to the present materials is asking for trouble. You might get away with it once, twice, maybe more, but the stress from smokeless on a gun originally set up and engineered for real black powder may well lead to serious injury and destruction of a gun that is just one of a limited number still in existence. All around a very bad idea.