Dmitry Bivol beat Canelo for a few reasons. 72" arm reach, strong stiff jab, faster hands, much faster feet, solid defense, he knows how to position himself to minimize what is coming back at him, he is patient and fought his fight and didn't let Canelo dictate anything to him, he has good solid fundamentals, VOLUME PUNCHING, good power. Also it doesn't help that Canelo is so slow on his feet I don't even know if he could have cut the ring off on George Foreman.
After mentioning everything I just mentioned do you know another fighter who possesses all the qualities in spades and then some: Errol Spence Jr. If you want to say Canelo is bigger that is fine, but from STRICTLY A STYLISTIC STANDPOINT Spence is a matchup nightmare for Canelo, Spence attacks the body far more often than Bivol does. Like I said Canelo is the bigger guy so he should be favored due to size over Spence, but from a stylistic standpoint it is nowhere out the realm of possibility Spence couldn't do the exact same thing to Canelo.