Dude, the UFC has one of the strictest drug/roid policies of any sport in the world right now. Guys have been banned because they had trace amounts of certain masking agents that can be purchased over the counter. A few have been disqualified/banned because the supplements they took were made in the same factory that was producing supplements that are banned by the UFC. They actually proved this by tracking down the manufacturers.
If this was 3 or 4 years ago, you'd be right. Overeem, Belfort etc. were all massive roid heads... It was obvious just by looking at them. Over half the guys in the UFC were taking something that The UFC was testing, but not really for cycles and for not as many substances. Now, they are extremely stringent... Lot of fighters have a fraction of the muscle mass they once did.