I believe that it was Johm McWillams, a very large BB'er around the early 50's or so, who had the first true 20" arm. He would sit in a straight back chair sideways, hang one arm over the back and just curl out a 100 reps or so. A lot of those old time BB'ers did a lot of rep's, I know. Leroy Colbert was probably the second guy after the 1950's to have 20" plus guns. Scott have 20"er's but only for a short period. He complained that his arm's actually hurt him all the time because of their size. Hard for him to sleep or even sit or walk around because of their bulk. I'm sure Sergio had well over that size. Lot of old time greats claimed 20" but were not much over 18 1/2 which is quite an accomplishment (in true muscular condition) .
Scott had just about perfect balance between the tricep & bicep center line, which can only increase their measurement with a steel tape. His arms look almost bigger than his waist in some of his shots. Can't say that now a days about most of the belly bloated Mr. "O" contest guy's.