I'm presuming that you are asking how much the largest part of the bore will open up with lapping, so will answer accordingly. If my answer isn't what you wanted, or my explaination isn't clear, fire another question till I get it right.
When properly done, lapping removes the constrictions completely with very little change to the major or largest diameter in the barrel. There will be some enlargement of the throat area if a lot of lapping is required to get the constrictions out, but the resulting taper puts a clamp on the bullets which tightens gradually till it reaches the major bore diameter. In other words, exactly what is required for top accuracy.
The amount of change in the major bore diameter will vary dramatically depending on precision of the barrel when lapping starts. If the bore is cloudy, which is rust standing up off the bore, the rust will wipe out with only a few lap shots and I've seen the slugged diameter increase .003 in 10 lap rounds. But the change is mandatory for cast to work as it is only wiping rust off. If the bore has a rough finish, lapping will likewise open the diameter as read by a slug, but again, lapping only takes the bore to what is demanded to make it shoot, by removing the roughness which files lead bullet bearing away, ruining accuracy. Unlike lead bullets, jacketed can slide along over a rough surface and shoot accurately, but the bore will foul badly, and lapping will minimize the fouling, while improving accuracy with jacketed also.
