I know that bash matters on your character size and your target size. As a Hell Baron (Large), I can bash Emtae/Colossus (Gigantic/Colossus/whatever they are) about 1/10 of the time. But, I could land a bash on Sirrus (Medium) almost every single time I try. Bash factors in strength, level, and size for the attacker and target, as well as the skill % of the attacker, if I remember correctly.