I'd disagree that it's a weight thing. I think the biggest impacts of the Raptor design on efficiency are its drag and higher rotating wheel/tire mass.
A Raptor has a higher frontal surface area than a regular F-150, and likely higher coefficient of drag as well. Regular F-150s have a chin spoiler, Raptors have large cutouts in the front bumper, larger wheel openings, larger A/T tires, all of which increase aero drag.
Additional rotating mass (90lb per corner wheel/tire vs. 60ish for a regular F-150) will hurt acceleration-related mileage, but won't affect cruise mileage.
And last but not least, Raptors are geared fairly short.
For a wild ass approximation of what a Raptor would do with an Ecoboost, start with a 2011 Harley or Lariat Limited with the 6.2L and compare that to a similar Ecoboost. Then multiply the Raptor's mileage by that factor. I wouldn't put much stock in the result though.