It doesn't work like that lads. Everyone knows they use steroids, it is IMPOSSIBLE to get that big without them, its as simple as that. You can reach your 'genetic potential' - but to get to their size, you have to use steroids.
The guys who compete in the strong man (an abbreviation for 'the worlds strongest man competitions') are huge guys as well but their dont train to be huge, thats not their goal. With bodybuilders, it is. Its about getting bigger and having every muscle in proportion e.g. same size biceps, lats, quads, calfs etc. This is why they go on a strict diet to drastically reduce the water rentintion in their body, so when they are ready to compete, it is just basically skin over the muscle, so they are completely ripped.
Afterwards, they eat, eat and eat carbs, carbs and more carbs to make up for the massive energy loss they have suffered. That really doesn't appeal to me in any way, its horrible being that big and just the lifestyle, the diet - no thanks. Because it becomes your life. But, on the other hand, its everything to some people. 
Also, ALOT of cheating goes on in bodybuilding, which for some people is a real shame when they work so hard to compete. I've seen it. 
 
I do quite admire a bodybuilders dedication, especially if he/she competes in the natural events (which there is - again proving that most of them, including the best, use roids).