I refer of course to the IFBB pros in oxygen gym.
Other then things that have been discussed like quality drugs, stress free environment, no distractions etc. I think a major reason is that they are surrounded by men most of the time. This creates for an incredibly anabolic environment.
So what do I mean? Well in a lot of mammalian life adolescent males are banished from the family group and then they away form their own groups or herds. This environment allows them to beef up and challenge dominant males.
The best example for humans is...PRISON. Despite poor food, training equipment and many stress factors, dudes are able to get jacked. I firmly believe it is because they are in close quarters with other males (no homo)