That's because it's a bigger plane with bigger wings and engines. It also has to burn more fuel to lift and carry the same load.
The 767 can fit in the same ramp space as the KC-135's it's replacing. It carries a little more fuel than the KC-135R and has better runway performance than the KC-135R. Considering our existing bases were designed for the KC-135, the 767 minimizes the costs of modifying the infrastructure already in place.
The burn rate isn’t that dramatically different with equal loads. Why do you think the A330 decimated civil 767 sales? Because it has a lower seat-mile cost. Fly both of the aircraft empty, and sure the Boeing costs less. Fly them both at 200,000 lbs of fuel, and they’re close enough to being equal. Fly them both with 240,000 lbs of fuel, and - oh wait, the Boeing can’t hold 240,000 lbs of fuel.
Yes, there is no getting around the fact that the A330 would take more ramp space and larger hanger space than the 767.
However, in forward airbases, the A330 can operate out of shorter fields while delivering the same amount of fuel as the 767. When operating in a combat environment, the A330 can carry and deliver more fuel, and stay on station longer, so fewer sorties will be required to deliver the same amount of fuel, an overall savings in time and number of aircraft required for the mission.
There are tradeoffs in all decisions.
Which configuration 767 are you talking about. They now have the 767-300er outfitted with winglets. If they are using that configuration on the tanker it should be able to take-off on fairly short runways. The ER was originally developed to allow flights to California to take off from NY Laguardia’s 6000 foot runways, fully loaded with fuel. I don’t believe that Airbus 330’s can takeoff from LGA.