Maddux not on roids. Look at him. Geez.
Not everyone who used steroids necessarily gets big muscles. Remember people didn't believe that Raphael Palmerio used steroids either because he was so slender and his strong denials. All the evidence was Jose Canseco's word on it until Palmerio tested positive.
The primary benefit of steroids to bodybuilders is the reduction of recovery time. Because steroids speed up the natural recovery process, bodybuilders can essentially double up on their workouts and reduce their rest cycles. This speeding of the recovery process is the same thing that pitchers desire. Pitchers using steroids don't need the same recovery time between starts as pitchers who don't use.
I have no reason to suspect that Maddux uses PEDs but I wouldn't be shocked to learn that any MLB player, including Maddux, used PEDs in his career.