The bigger engine makes more torque down low, so it's better optimized for highway cruising when in the proper gear. Weight makes less difference at highway spe

The bigger engine makes more torque down low, so it's better optimized for highway cruising when in the proper gear. Weight makes less difference at highway speeds, as wind drag is the biggest factor. Both your rigs are big boxy SUVs and so the aero probably isn't much different. Hence the torquier engine being more efficient on the highway. This is why Corvettes can get 30mpg and still have 500hp engines. They make the same torque at low cruising RPMs as they do at higher RPMs. The engine just lumbers along at low speed, while a smaller displacement engine is going to be working harder to break through the wind.