If Sergio's were 21" at 5'9" tall,Arnold coudn't have less than that.
Looking at the pics Arnold and Sergio looked to have about the same arm size IMO. Oliva's were 20.5 cold, probably close to 22" pumped, about the same as Arnold give or take a little depending on bodyweight.
I believe Arthur Jones on Oliva's measurement but not on Arnold's, they were about the same size.