Velocity is obtained by applying force to the base of the projectile. The velocity is of course proportionate to the amount of force as well as the area of the base.
The areas of bullet bases are relative to the square of their diameters. (.72/.45) = 2.56. Thus if you had an exactly identical pressure curve a 235 grain bullet would be pushed to the same velocity in a .45 as a 600 grain one in the .72.
A 500 grain bullet (old government weight for the .45-70) would have to reach 1500 fps to develope 2500 ft/lbs, a 600 one 1370 fps. It would take a good bit more pressure to get that 500 grain bullet to 1500 fps from a .45 caliber barrel than to get the 600 one to 1370 in a .72 bore.
