The "Magnum" cap carries 15% more, by weight, of the primer composition compared to a "standard" cap. In the case of Remington Black Powder caps they contained 50% more, by weight, of the primer composition.
Black powder is best ignited by flame or particulate debris heated to incandescence. Heated gasses have little effect in igniting grains of black powder.
The larger the grain size the more difficult it is to ignite black powder with percussion caps. The larger the grain of powder, the thicker the glaze is on the grains making it more difficult to get through the glaze to the combustible portion beneath the glaze.
I went through all of this with a guy from the Netherlands who spent a week at the Hagley Museum working with antique powder testers for a book he published on them and how they worked.
When we tried percussion caps with a cannon powder in the old powder testers we could not get a single ignition. The grains came back out looking like glass beads. None of the caps produced sparks large enough to melt through the heavy glaze on the powder grains which would have given ignition.
Mad Monk