Foreman, Duran, Pedroza on PED's?

I hear one of the main examples on why people believe Pac is on PED's is that his punch resistance improved. That he took Cotto best punches and smiled. So does that mean that we should be suspicious of Foreman, Duran and Pedroza? George Foreman gets stopped by Ali and dropped by Jimmy Young in his prime, comes back and is walking through punches like the Terminator. Roberto Duran gets dropped at 135 twice by Esteban Dejesus. He fights Hagler at 160 10 yrs later and Barkley at 160 17 years later and neither guy was able to put him down. Can anybody say DeJesus was a bigger puncher than Hagler or Barkley. Eusebio Pedroza was one of the best featherweights ever. But before that he was stopped 3 times at bantamweight. All guys who got dropped or stopped at lower weights, went up in weight and showed a good chin.