Uh, are you really saying that big black people are not victims of racist abuse?
It sounds like he's saying "people are more likely to intervene if they think they could take the other person in a fight". If you're afraid because of a size differential, you're more likely to stay silent (not "you" specifically, but the societal "you")
I felt 70% of the reason why the Hispanic guy was confronted by people was because he was a much smaller than the white guy.
Had it been similarly sized black guy, nobody would've dared scolding him.
It's socially unacceptable to be racist, but we think being 'sizist' is ok.