I think the OP is referring to a pistol and not a revolver.
But just in general concerning revolvers – YES, the throats of a revolver cylinder are the critical factor in that system. On a revolver cylinder, the throat at the forward end of the chamber is the last sizing die the bullet will go through before reaching the barrel.
When dealing with revolvers there is no sense in sizing the bullet larger than the throat diameter because the bullet will be “sized” by the throat before it reaches the barrel. And there is nothing to be gained by sizing the bullet smaller than the throat, unless you just like poor accuracy, lost velocity, and removing leading from barrels.
In a perfect world………the throat diameter will be slightly larger than the grove diameter of the barrel in front of the cylinder. In that situation, the best practice is to size the bullet to match the throat.
NOW, getting back to the OP, who I believe is inquiring about a pistol barrel with a 40 S&W chamber, he has a different set of parameters.
First, the cartridge must reliably feed into the chamber. Because the 40 S&W headspaces on case mouth, the “throat” is effectively the area just in front of the chamber before the rifling begins. The amount of space here determines how big the bullet can be in both diameter and length beyond the casing.
Second, the bullet must fit the barrel (which is why he is concerned about groove diameter).
Here the criteria are a bit simpler. The bullet needs to be fat enough to fit the barrel but not too fat to go down the barrel.
Fortunately, when dealing with pistol chambers that are integral to the barrel (as opposed to revolver chambers) you only have ONE chamber to deal with.
For a pistol barrel (not a revolver barrel), a bullet seated in a casing that will reliably enter the chamber and that is slightly larger in diameter than the groove diameter, is all that is needed. 40 S&W chambers/barrels are fairly consistent. The SAAMI specifications for a 40 S&W barrel are a bore diameter of .390” and a GROOVE diameter of .4005”. So, a cast lead bullet sized to .401” that will reliably chamber, is probably going to work.