Your hitbox isn't the problem - as far as the game is concerned you and the dwarf are the same size.
We haven't downed Anub heroic, but I (gnome) solo tank the adds and don't have a problem with them being behind me.
I would suggest you and he changing locations, but keeping all misd's, positioning the same, a small difference in angle of mobs may be the issue - are your misds coming in sooner than his, causing the angles to be different? OK just reread your thread about no misds etc, in that case what is your latency like compared to him - are you battling to keep them ahead of you because what you see is not what the server sees?
My main problem on Anub was disappearing into some of the paving stones and suddenly getting a screen that was completely grey