Volume can't be more. The volume of the bore stays the same regardless of powder used, although the volume increases as the bullet moves down the barrel.
At, for example, 55,000 PSI, the compression of the gas in the barrel would be the same with either powder, but since the larger amount of 4350 produced more gas (and is possibly still producing) the pressure on down the barrel does not decrease as rapidly as the volume increases as the bullet moves forward, increasing the bore volume as it travels.
However, this theory comes apart if you consider that the peak pressure of 55,000 PSI for either 4895 or 4350 remains at 55,000 for the same interval.
