I agree on one hand, most fighters don't do well going against heavier guys, and that seems obvious. The exceptions are the likes of Pacquiao who like to exploit slow footwork and movement, which is why a fight like Margarito is not as much of a challenge as a quicker guy a similar size like Bradley.
For Canelo though, I do think he will benefit from fighting bigger guys like Lemieux. All of the struggles we have seen from Canelo thus far have come against, smaller, quicker guys. Lara and Floyd being the two obvious ones. Even Khan caused him troubles initially because of the speed and movement. His feet aren't very fast so when he's fighting someone with slow feet as well, the gap that was there when he tried to hit Floyd, Lara etc isn't there, and he tee's off much more comfortably.
I understand what you are saying. But at the same time, Canelo has gotten by those smaller guys like Lara and Khan (in part) by having superior size and fire power. Lara had no interest in absorbing any punishment from Canelo, and that is a big reason why Canelo was able to outpoint him. Lara was on his horse the entire fight. Khan physically couldn't take the punishment, which is why he did the chicken dance every time he got hit and eventually got knocked out.
A guy like Liam Smith, who is small and relatively easy to hit was able to have some limited success, and last quite a while with Canelo. Eventually he just couldn't take the punishment. But if Liam Smith was a natural middleweight, that's not such an easy fight. I saw an interview the other day where Liam Smith said that he thought he was the same size as Canelo at the weigh in, but when they stepped in the ring he looked huge.
Canelo cannot take shots from Jacobs, GGG, and Lemieux the way he took shots from Kirkland, Cotto, Lara, Smith, etc. These are much bigger men he's fighting, and they are also p4p some of the harder punchers around. Their offense will pose a real risk to him, unlike anything he's really dealt with so far.
Canelo's best attribute at middleweight may end up being his defense, honestly. He's a very good defensive fighter, probably the best in the middleweight division (when he fights there). He's not going to blow through middleweights like he did Kirkland, Khan, and Smith.