Just to add some history- .38 Special, born in 1899, originally loaded with 21 grains of black powder. That is why you have the volume of a .38 Spec case- black powder had to completely fill the case right up to the bullet.
But when they changed to smokeless, it fills only a small percentage of the case. And unlike black powder, it is ok to do that. It would be POSSIBLE to put about 8-9 grains of Bulleye powder in a .38 Special case- a VERY DANGEROUS triple overload.
When the .357 Magnum was created in 1935, they started with a heavier framed handgun, a catridge case just a smidge longer so you could not accidentally put it in a .38 Special- and then they used the available space in that cartridge case.
And no, gunpowder is NOT gunpowder- there are different powders that have different burn rates, different pressures- and it is rare to have a smokeless powder that completely fills the case.
THAT is why reloaders have scales, and reloading manuals- you have to match the correct amount and type of powder to the weight (and design) of the bullet for THAT caliber- otherwise you will destroy a gun, hurt yourself and innocent bystanders.