The full classic story of Cannonball Adderley's nickname is that he was at first called »Cannibal« because, when they were young, he was so fat and his brother (Nat, a noted trumpet player in his own right) so thin that it seemed as though Julian must have eaten most of Nat (or so their friends would claim). It was subsequently misheard as »Cannonball« when he began to meet people who didn't know his brother or their background, just that Adderley was in fact very fat. Since »Cannonball« makes much more immediate sense and is also honestly a pretty cool nickname in its own right, that's what stuck.