are you sure it isn't just that he wears tight t-shirts and they are proportionately big? meaning, he is pretty little, so decent sized arms on him will look bigger. whereas if he were 6'3" his arms would look tiny?
don't know - but I know for me, if I wear a tight shirt, people will always say my arms look huge, and I don't have big arms. at best 17" but I'm not sure, I haven't measured in quite awhile.