"magums" have larger case capaicities and is able to ultilize the slower powder better by building pressures slower. You can load a 300RUM with IMR4350 fine, you just might not be able to get the desired velocity out of it where you can with RL22 or Retumbo. You can load a WSM with Retumbo also, but might not be able to reach its potential velocity with it. A more suitable powder might be IMR 4350. As case capacity changes, your pressure curve changes.

Most reloaders try to find a powder with at least 90% load density. Where with a slower powder you might have a compressed load, >100% load density, but still not reaching peak pressure, there fore probably not reaching potential velocity. And funny enough, its not just the area, but the shape of the area. 60gr RL22 will give you sizzling velocitys in a 270Win. where RL22 might not do the same for WSM but RL19 would be better.