Well gentelmen, I am an engineer and part of the problem is that we are not dealing with a set of constanants. The variable is the diameter of the bullet, therefore everything becomes somewhat of a compromise, so we eliminate the variables that we can.
To get consistancy in where the bullet lands on the target, each bullet should experience the same pressures, constrictions, etc as it exits the muzzle as it would if fired from a single shot or autoloader which deal with a single chamber, throat, forcing cone and barrel.
Opening the throats makes sure that the bullet upon leaving the forcing cone is not smaller than the groove diameter.
Making the throats a consistent size, assures that each bullet enters the barrel with the same amount of pressure behind it and at the same time in the recoil cycle.
And yes, too big of a throat can also cause problems as seen on some pre-war Colts. Although in the example I gave earlier, with the .451 FMJ, you would be very hard pressed to see any light between the bullet and the wall of the throat, so it cannot tumble.
One final thought, for the best groups it is important that the shooter keep his eyes open when he pulls the trigger!