I'm struggling with lalimes Martin's take there, the turnover had nothing to do with physicality or lack there of, Brannstrom just played it really poorly.
Puck hung up on the boards on him, he left space between the boards allowing the F1 to get inside position on him, he actually recovered well, winning the puck battle with the F1 and chipping it behind the net before F2 arrived, but he put it too far for Guinette to get who had coasted in ostensibly to provide support. Because Guinette wasn't there to provide the outlet option for Brannstrom, the puck went to the far side where F3 collected the puck.
But let's blame it on on Brannstrom being too small...
Very good structural analysis, but I think you are taking physicality off the table a bit too much.
While everything you've said was accurate, a bigger/stronger player has a safety net when they play things poorly like Brannstrom did. I am not even talking a behemoth... guys like JBD and Thomson are short for DMen but still have size & strength on Brannstrom (or, at least, will have strength on him).
No one is saying Brannstrom is too small to succeed in the NHL, because there have been DMen his size and slightly larger who have had tons of success at the NHL. It is the combination of mediocre decision making, mediocre skating (not for an NHL, necessarily, but certainly for one of his size), and being small and weak that combine to make him a difficult player to be fully comfortable with.
So while I think you are right, I also think his physicality will always come into play because it forces him to rely on other facets of his game that are just OK.