Had a recent axe spec wielding 25lbs axe and 15lb axe had a scuffle with an arial assassin who just had a dagger in offhand parried my axes with ease.
Not sure what is classed as heavy for it to kick in.
Also surprised how well he parried axes in the first place. Perhaps at higher ranks with weapon specs and such he would not have parried a weapon he doesn't know as well.