A bit of history on the .45 Colt cartridge and chamber.

It was introduced in the M1873 Colt Single Action Army revolver and adopted by the US Army.

Being a black powder cartridge, it was determined that chamber and throat tolerances would be on the "loose" side for reliability in sustained fire (combat) situations. 0.454" bullet, throat, and bore diameters were on the slightly generous side to allow for accumulation of firing residue and to allow for continued firing under conditions that did not allow immediate cleaning and lubrication. Chamber throats will routinely be found to be 0.456" and LARGER in both Colt and S&W products in .45 caliber. This was a routine production expedient during the black powder cartridge era (1870s thru WWI).

During WWI, S&W N frame revolvers were adapted from the previously chambered .45 Colt to .455 British caliber for British Army requirements. .45 Colt throat, bore, and groove diameters were used because they were very close to the .455 specifications.

When the US Government assumed control of S&W M1917 revolver production, it was determined that .45 ACP proof pressures were significantly higher than that of the previously chambered .455 cartridge. Because of the urgent need for M1917 service revolvers, the decision was made to make chamber throats to the previous .45 Colt and .455 British dimensions, namely 0.454"-0.456". This resulted in two things: increased combat reloading ease, and ability to pass the required .45 ACP proof test without the increased strength of heat treated cylinders.

In a nutshell, that is why S&W revolvers chambered in .45 ACP seemingly have oversized (.45 Colt sized) throat diameters.