I have long contended that the reason modern over/unders are popular in 20-bore is that a typical 20-bore O/U weights the same as a typical 12-bore side by side: that is about 6 1/2lbs.
This weight and handling dynamic clearly suits the majority of game shooters.
I think the small bore fad on the vintage gun collector scene is motivated by a number of things. For some it is willy waving in reverse - "mine is smaller than yours, which makes me better". Never mind that so many try putting 12-bore loads down 20-bores, they think they are more skilled and sporting because they use a small bore gun.
Another reason is rarity. Collectors like to have something their friends don't have. As there are fewer small bores of quality, having one is a sign of success and wealth etc in the mind of some owners.
Some people just like carrying light guns, especially as they get older.
Me? I prefer a 12-bore hammer gun to anything else. I also shoot quite well with a 16-bore hammer gun. I find I shoot 20-bore or anything smaller comparatively badly, I just don't like the handling characteristics of very light guns.
