Cena, Mason Ryan, Sheffield, Big Zeke, etc. all do indeed have a portable gym, its called Dianobol, HGH, and every other drug under the sun. There's simply no way to stay that big (& lean) naturally while traveling 300 days a year, taking bumps and injuries and having to eat on the road. As a (natural) bodybuilder myself i'm telling you for a fact that's not possible.
Now, i'm sure some of them could stay pretty big without juicing but they'd be a lot fatter with less muscle definition. The steroids are why guys like that are huge but also look cut at the same time. When you bulk up you gain more fat than muscle, its not really possible to stay huge with sub-10% body fat, visible abs, etc 365 days a year. As someone who follows professional bodybuilding closely, i'd wager the majority of the roster are taking something.
What you need to understand is steroids arent just for getting bigger, theyre also used for losing fat and staying lean without proper diet/cardio, and recovering from injuries fsater. Being on the road and wrestling 4-5 days a week means you don't have time to do cardio and certianly can't eat 100% cleanly. A lot of them are working hurt and have no vacations. Why do you think guys like Rey Mysterio get busted for using them? He has a lot of injuries and no time to rest and recover, steroids make you heal faster. Cena is 100% certainly taking them too, cause he's working more hurt than any wrestler in history.