The biggest advantage a tall fighter has is the ability to just pull his head back into a safe zone. But Bena never does that. He relies mostly on blocking shots with his gloves, plus some lateral movement. Canelo will have all the advantages that a shorter fighter always has, with the most relevant one being that it's harder to throw an uppercut against a much shorter fighter. One of Bena's main weapons is the uppercut.
Bena is also somewhat susceptible to body shots, as he's tall and likes to keep his gloves high. Canelo's great and throwing hooks to the body, and being so short, it will be even harder for Bena to see them coming.
And finally, while Bena's #1 advantage is his exceptional hand speed, Canelo has the best defensive head movement in the sport today. If anyone can avoid / diffuse Bena's standard attack, it's Canelo. That means Bena will have to rely more on slower power shots, and that means fighting closer, where Canelo rules the land.