I believe that the tradition of oversized cylinder chamber throats is a leftover from the blackpowder days. Under the old blackpowder ammunition, it was actually advantageous to have oversized chamber throats because the shooter was able to continue firing even when the chambers were badly fouled from firing residue. It was considered to be more important to be able to continue firing than have absolute accuracy from aimed shots.

So, why did this tradition in gun manufacture translate into having oversized throats in the smokeless era? Wartime manufacturing contingencies were probably the reason: transition from making .455 to .45 revolvers and accuracy "good enough" for close combat plus the ease of reloading in dirty trench conditions, plus the fact that S&W never discarded any gun parts. All guesswork on my part, but I've measured a lot of chamber throats and the results support this theory.