In theory you are correct, if you were to take MPG measurements during a highway trip that does not include getting up to speed, and only while at constant speed, assuming your odometer is correct, or you are using another method of measurement, you will increase your MPG. In reality though, it is all about the weight and the fact that you are changing your final effective gear ratio. However not as severe, here is the idea, with larger tires it is like trying to take off from a stop light in 2nd gear rather than 1st. The final effective gear ratio is now higher and thus requires more fuel to get you moving. This is the reason a number of people regear after going to larger tires which tends to get some of the gas mileage back and lowers your final effective gear ratio. Your crawl ratio is also effected with larger tires, you are not able to crawl as slowly in 4lo and 1st gear. The severity of these effects are directly correlated to the difference in size from the larger tires to the stock sized tires. The larger the tire the more significant the effects. This also affects the measurable torque at the tires as well and you will also require more power to get up large hills. Usually off road types tires also have a greater rolling resistance that the truck must overcome to cruise down the road. All this combined is responsible for reduced MPG.