There is code in Client:: Damage which cuts all PVP damage to 2/3 of normal, so that is probably causing it.
Simple fix would probably be to change the code in question to this:
Code:
//Don't do PvP mitigation if the caster is damaging himself
if(other && other->IsClient() && (other != this) && damage > 0) {
int PvPMitigation = 100;
if(attack_skill == ARCHERY)
PvPMitigation = 80;
else
PvPMitigation = 67;
damage = (damage * PvPMitigation) / 100;
}