First, those people in the pics you posted probably take anabolic steroids. That's how the majority (not all!) of bodybuilders do it. Those that don't take steroids must do tons of workouts, yet they cannot approach the steroid users in terms of muscle mass.
Second, the only way you gain fat is by having high levels of insulin. You can have high levels of insulin if you regularly eat lots of carbs, which I assume you don't since you are on this forum.
Third, the way that muscle mass is depleted by the body is the mechanism called
Gluconeogenesis , and that only happens if you are starving (i.e., your body needs to generate glucose for the brain and the only source is your muscle mass). This is extremely rare and as long as you have enough fat intake, nothing to worry about.
Fourth, and this is more my personal opinion, is that the scenario of bodybuilders gaining lots of weight after not working out is caused by continued intake of high carbs (often deemed essential by athletes, even though they are not) plus the stoppage of steroids. Bodybuilders often eat a lot of carbs, even while claiming to eat high protein (you can't eat but a certain percentage of protein, so the rest is either carbs or fat, and most bodybuilders mistakenly opt for the carbs). It's entirely likely that once a bodybuilder stops working out intensely, he or she believes that she should stop eating so much (true) and cuts back on the protein and fat first (wrong).
Good Luck, I hope you keep on working out!