Modern aircraft have bigger, high BPR engines, which in turn weigh more. This increases OWE but it's worth it for the fuel burn advantages. That said, compared to some of the really old models engine weight might be ~neutral as structural enhancements will cancel out the extra size.
With all of these /seat calculations though it depends heavily on seat density. A high-density config will nearly always beat a long-haul config with business/first seating, regardless of the actual structural efficiency. Perhaps a measure of OWE per unit of cabin volume would better? But again this obviously wouldn't take into account fuel.