Young guys are working out like mad these days (and hitting the ball miles). There is no doubt in my mind that there are performance enhancing substances being used in golf. It's not just steroids--there is a wide range of substances that can give you an edge. There can be many benefits to such substances, but one of the most common is that you can work out longer, and more often, thereby increasing your strength, speed and quickness.
Not all of these substances bulk you up. Look at competitive cyclists. Those guys are built like broomsticks, yet their sport is riddled with performance enhancing drugs.
Golf needs to get ahead of this and start testing ASAP.