it wont just be oil pump, but yes that effects it obviously but oil pressure will be influenced by many aspects.. bearing condition and clearances, avcs heads and pump size.. there are 10mm, 11mm and 12mm pumps.
My 6cylinder has 2bar idle and 4bar on throttle (fairly normal for an EG33).. its got more bearings to run the oil round so more have leaked out and lost the pressure
Simon