I think the difference is that reddit has more social network lock in, which allows mods to be bigger assholes. People will tolerate asshole subs if the sub is big enough.
Second issue is subs are more likely to BECOME assholes on very large subs because it's a lot of work herding kittens.
Third issue is mods are ALREADY likely to be assholes because modding large subs is very much not fun and an insecure ego is a much greater motivator than secure altruism.