Never had a velocity issue with my 20g with standard 12g loads. I don't do fairy powder charges either and never thought I was not being safe!
In the past twenty years there has been a trend for cartridge ammunition to get lighter and lighter loads. Many an option for 7/8oz loads exist for 12g!
It's just popular choice, there is no rhyme or reason to it. " Yeah I got the 12 instead of the 20".
"Really, why Bubba".
"Well, cousin Jo says it's better so.....".
How is it better?(don't answer).
How many times has shot bounced off a critter from a 20g?
How many less pellets have you had to pick out of meat from using a 20g, or how many more pellets have you picked out using a 12g?
It's all jibba jabba nonsense.