This is why we sell more full size trucks than midsize trucks. The fuel economy is basically as wash so there is no real benefit unless you are buying one because it costs less, live in a metro area where vehicles size matters, or just like smaller trucks over full size trucks with more room.
My father in law found this out the hard way. He bought a 2009 Toyota Tacoma for fuel mileage for then he doesn't need his F350 to haul stuff. That truck gets worse if not the same hand calculated mpg than his old 6.0L and current 6.7L F350.