A road train is always going to be bigger, so self preservation tells me to pull off the road. That, and if we both pull slightly off the road to pass each other my 3 kerbside wheels might kick up a rock or two, but his 25-odd kerbside wheels will kick up many many stones that will chip my windscreen/bonnet/headlights.
Rules may be rules, but it just makes sense to get out of the way!