20s and 16s were never built for recoil issues. There were built simply because they are lighter and thus easier to carry. They recoil about the same as a 12ga because of gun weight differences. Some 16s, the ones built on a 20ga frame are quite stout in the recoil department.
I call nonsense on that. Take a 12 ga slug and compare it to the others, the 12 will rock considerably more because of both a heavier slug and powder charge. Use something as close to as apple to apple as possible say like a cheap single shot Topper. I'm not sure BUT I think the 16 was a Euro bird shell and never really intended for heavy use.
http://www.chuckhawks.com/shotgun_recoil_table.htm