Tell me, kyomu-san: U think he has used little amounts in the past 3 years? 
I dont have any clue.
First of all, many people underestimate our natural power and very arrogant.
Before talking about the natural limit, how many people have dedicated super hard training and strict diet sacrficing the general life things at least whole year around?
As far as i know, even many pros dont dedicate their hard training and diet whole year around.
So, no wonder that a pro makes significant improvement without increasing the dose if he dedicate bbing life style to the death whole year around.
But again, i dont know if Dan did it or not.