If that was the case icing reports would Vary. ie moderate to a 777 pilot may be severe to a light twin pilot which obviously is not the case in PIREPs
Actually, that is precisely the case. One must ALWAYS consider the source aircraft reporting the icing, as the nature of what is being reported is VERY aircraft-dependent.
Severe icing is always icing that exceeds the ability of the airplane to contain or remove the ice accumulation; it's ice beyond the specific airplane's capabilities to handle the ice.
Icing reports do vary; absolutely icing reports vary. This is ALWAYS the case with PIREPS. Moreover, the nature of what's reported is subjective according to the individual experience level of the pilot. What is light to one pilot may be moderate to another, even in the same type aircraft. What is light in one type aircraft may be moderate or severe in another type aircraft.