Please post your source showing that Rodgers KO'd Lesnar.
Overeem has a slight chance against Brock, but even with all the drugs, he is still much smaller.
Like Mindspin has stated before, Brock naturally carries the muscle he has. Many of these other guys have to work and lift specially just to carry it.
Big difference in cardio when you do this.
Bottom line, Overeem really has a small chance of beating Brock.