This is more reflective of how many games their teams play, and how much better those teams are than the rest of the League. On a goals per match basis, both Messi and Ronaldo's best La Liga season is inferior to Zarra's best seasons, and comparable to Hugo Sanchez's or di Stefano's best season, and slightly better than the real Ronaldo's, Romario's, or Eto'o's best seasons. Frankly, Messi and Ronaldo are as good as all of the above except perhaps di Stefano (who the old Spaniards say was the best). If you throw in all matches (including European leagues), Puskas and di Stefano look as good as Messi, and better than Ronaldo, on a goal per match basis. Of course there is more to it than goals, and Ronaldo plays alongside Higuain and Benzema, who together, splitting time, score as much again as he does (!), while Messi does more than score goals.
While the real Ronaldo and Romario certainly took some drugs, both the good king and the bad kind, somebody like Puskas certainly did not (Puskas was famous for being fat and refusing to move much). So scoring loads of goals in and of itself is not indicative of doping.
I see no reason to think Ronaldo dopes more than John Terry.