This reply is a little late. Cartridge crimp is the answer.
When you reload a cartridge and you crimp the bullet in place, pressure climbs and or "spikes" on ignition until the crimp releases from the canelure. That very brief moment causes the pressure inside the case to "spike" radically, thus making any 15 or 20 grain powder load produce a higher pressure curve briefly in the process.
Another factor is bullet diameter and shape in the cartrige situation is radically different from a greased and seated round ball. Only the "equator" of the ball so to speak will actually contact chamber and barrel, thus making a smaller surface insofar as projectile compression in the forcing cone and actual contact surface of the projectile to the barrel. It takes less pressure to push a greased round ball out of a chamber, through a forcing cone and down a barrel, than it would to force a cylinder of lead out of a crimp, into a forcing cone and down a barrell. The job becomes even more difficult should you add a copper or brass jacket to the slug seated in the cartridge case. Internal pressures climb rapidly when there is more resistance to the forward motion of the projectile through the process of the weapon.
Another factor to consider is the ignition primer. Cartridge primers can be much hotter than BP caps, and this plays a factor on powder burn and pressure curves.
For these reasons, it is safer for the manufacturer of the replacement cylinder to state "steel frame" or something to that effect, from a liability standpoint.
In matters of pressure, you can make a simple 9mm parabellum cartridge spike dangerous pressures simply by pushing the bullet inward, over a factory load. It will actually spike dangerously enough to damage a firearm, even though powder capacity has not been increased.
Many things will affect pressures without increasing powder. Even ambiant temperature will have a factor on powder ignition and pressure curve.