bodybuilders generally need to be looking ripped all year round so basically never come off them.
No that's not exactly accurate.
First, in bodybuilding parlance, ripped = lean. Although I think the average layman uses it to mean "muscular" But having muscle mass has nothing to do with being lean. Professional strongmen, powerlifters, football linemen etc. often have enormous muscle mass but high body fat levels. They also take a lot of steroids as well.
Would like to point out, most guys compete only a few times per year. The demands on your body of dieting down to 5 percent body fat or less while training heavily is very stressful on the body and that condition cannot be maintained long. It's called "peaking" and most guys can't tolerate it with much frequency. 90 percent of the pics you see in magazines are made within 6 weeks before a contest. For 95 percent of guys, the body cannot hold contest conditioning for more than a few weeks at best. Steroids do not help you get ripped as much as dedicated fat loss drugs (Clenbuterol, Thyroid, DNP etc) but they help you KEEP the muscle you already have while on caloric restriction.
The main reason bodybuilders take anabolics year around (during the off season) has to do with the constant pursuit of more muscle mass. To maintain and grow ever more muscle beyond the body's natural testosterone levels require the body to be bathed in androgens as much as possible. Plus steroids let you recover faster from injury, lubricate sore joints etc. Once you return to normal human androgen levels, eventually your muscle mass is going to return to normal human limits as well..
You didn't say this explicitly, but if you think Brock is the only MMA guy on performance enhancing drugs, or that Fedor has never used them, I have some swampland in Florida for ya

My guess why Brock looks smaller has to do with the fact that his training is now totally geared towards functional MMA fighting, not visual appearance. Thus he has dropped excess muscle through long and rigerous endurance training. I bet he's still on various drugs though (but perhaps at lower dosages). Any time you have a professional/competitive sport where enhanced muscle mass, strength and recovery is helpful you will find steroids and other drugs widely used despite drug testing. That goes for all major sports and the olympics as well.
As for Fedor vs Brock.. I think it would be a great matchup, but what Fedor gives up in size and power he makes up for in experience and training. Brock is new to the sport (4-1), whereas Fedor is an old pro (30 -1). As has been repeatedly proven in MMA - size and power are not the be all end all of combat. MANY huge guys have been slaughtered by smaller men. A lot of people assume Fedor would doninate Brock since he has such an amazing record, but ANYONE can be beat.
EDIT: Just realized this is way more than most people care to know about the subject, but what the hell.. I'm very familiar with the topic so why not..