Well he has said that more recently...
http://www1.skysports.com/tennis/ne...doping-controls-in-tennis-have-to-be-improved
http://www.heraldscotland.com/sport/tennis/murray-lets-flush-out-the-drug-cheats.19282031
I have a lot of concerns about Tennis, but I don't see that his body has built up particularly fast. Most full-time athletes should be able to build that amount of muscle at that age with a dedicated fitness trainer helping them. If he is doping, then he's not doing it very well, as I thought he looked tired throughout the Australian open final, breathing very heavily after only one set, as if he had not recovered from his semi-final.
Djokovic on the other hand seemed to recover superbly after a much longer match against Wawrinka.
Sadly his dedicated fitness instructors appear to have track records. So yes I have doubts.