It's a common misconception when buying something with a small 4 cylinder engine that it will get better MPG than something with a 6 cylinder engine instead.
While the smaller engine is usually advertised at a higher MPG figure than the 4 cylinder, that same vehicle with a 6 cylinder engine in it may get equal or even better MPG than the 4 cylinder.
Reason being the 4 cylinder has to work harder to move the vehicle where the 6 cylinder works easier.
All bets are off if you are lead footing it.
I have an Acura MDX 7 passenger SUV with a 6 cylinder that is 270 HP and I have gotten 33 MPG on the road with the norm about 25MPG but around town with stop and go, it is about 14-16 MPG depending on the season (A/C) . There's a lot of smaller cars that can't get that kind of MPG even with a small engine but there are a few that will, problem is with them that you can't fit in them