Actually, a large-displacement engnie that is appropriately cammed for low RPM torque, can get surprisingly good MPG numbers. The LS1 and the 3800 V6 come to mind as engines that out-MPG smaller DOHC engines, when moving a similar load.
Look at trucking, where MPG isn’t everything…it’s the only thing. LARGE displacement for the HP they make, even for Diesel engines. That’s because low RPM/large engine tends to outdo high RPM/small engine in fuel efficiency.