Well you should rest easy knowing that Byfield dominated in many ways, but "because of his size" is not one of them. He doesn't play to his size advantage in the fashion that your fear would become justified.
That's not true.
He doesn't smash guys very often, but he constantly uses his size/weight to protect the puck and bull his way to the goal.
So, yeah, I think there's a ***small*** amount of justification to the idea that Byfield's stats are inflated and that the stuff he does in junior won't translate against 6'2, 210 30-year-old defensemen in the NHL.
On the other hand, despite his weight, he also looks like a baby giraffe out there sometimes. His weight distribution clearly isn't ideal for hockey yet. He hits guys and Byfield falls, because of that weight distribution.
When Byfield hits his ideal weight, when that weight is in the right places, and when he learns how to use it effectively... even if he never develops that elite-level hockey sense... he's going to be a force in the NHL.
He still has the potential to be by far the best player in the draft. He could be a 6'5 Kopitar with better wheels.
But he might also top out as a physically dominating 60-point center.