Ali had 15 inch arms. Frazier about the same, maybe a slightly smaller. One of the reason fighters arms are a lot smaller than many think is due to lack of tricep development. Large triceps for a fighter is a hinderance, causing fighters to "push" their punches.
If you want to see how muscle mass hinders fighters, just look what happened to Frank Bruno, every time he fought anyone above B- class fighters.
The Tyson one showing his arms at 16 inches they didn't. I remember the HBO preview when they did it and his arms were hanging unflexed by his side.
From TOT measurements I've ready on most WW boxers recently: Margarito, Mosley, Pac, Cotto, Floyd, the measurements seem to be around 13"-14" Simply google: "Pac vs margarito tale of the tape" "Pac vs Mosley tale of the tape" etc., etc. . .