It is not the diameters, but how much smaller the diameter of the restriction is compared to the bore of the shotgun barrel.
Here is a very good explanation, from the Briley website:
Understanding Shotgun Chokes, A brief explanation by Briley
Here is a quick reference chart:
Shotgun Choke Yardage Shotgun Choke Restriction- difference between bore and choke
Cylinder.................< 20..................0
Skeet.......................22.5................005 of an inch
Improved Cylinder.....25...................010
Light Modified...........30...................015
Modified...................32.5................020
Improved Modified.... 35..................025
Light Full...................37.5...............030
Full...........................40 or More.....035
Extra Full...................40 or More.....040
The website I linked at the top has a very good explanation, so it would be informative for you to study it briefly.
Regards,
Mark
P.S. Welcome to the forum.