Even if a person's size could be construed as a weapon, that doesn't justify lethal force. What happened to tasers? What happened to pepper spray, batons, beanbag rounds, etc.? Why go immediately for the kill?
Not that I'm advocating any of those. To me, Brown's death is pretty clearly on the spectrum between manslaughter and murder. But generally speaking, it seems like less-than-lethal-but-still-violent methods are fading away.
In Durham a man was recently killed by police in an obvious suicide-by-cop situation. They were blocks from the police station and the standoff lasted more than an hour before one of the police killed him. It would have been a perfect occasion for a bean bag round. And a bean bag round would have reduced the danger posed by gunfire in downtown Durham.