Does size matter in a fight? That’s an age-old question in the combat sports community. I believe the answer is a resounding yes. Sure, technique can overcome size (i.e. Royce Grace in the early UFC days), but if you match two people of roughly equal skill, the bigger guy is going to have a huge advantage.
The video shows a training session between two muay thai fighters. The smaller guy is Saenchai, who is regarded as one of the greatest muay thai fighters of all time. Even with his honed skill, he was completely out clenched by the bigger training partner.
There are weight classes for a reason. Size IS a determining factor when all else is equal.