Asuma said it himself on his death bed that Chouji would become the strongest of all of them. Typical shounen element when character taps into their deep emotions their power level goes way up.
Chouji had the potential he just lacked the confidence thats what the flash backs where all about Asmua & his father all telling him to be confident. Well he just found his confidence and his reason to fight, something Naruto and Shikamru already found. His Nindou way so now his true power has awakened.