The AMC/Tanker side of the house thinks a bigger tanker (aka ... more fuel more cargo) is best. It makes sense, bigger is better. Sometimes.
However, when you start comparing them to the mission they're replacing and the types of aircraft they're refueling, it doesn't make sense.
A bigger tanker requires more ramp space. Any overseas conflict is going to have limited ramp space. So if we buy bigger tankers, that means, we can't bring as many to the fight. The result, less booms in the air. And there's the problem.
Fighter guys don't care about how much gas is in the air. The USAF can almost always put enough gas in the air. We can not always put enough booms in the air or booms in all the right places. To do that, we need a smaller more tactical airplane like the KC-135. A larger more strategic airplane like the KC-10 or (A330), doesn't make sense as a replacement for the KC-135.
So if you replace 2 KC135s with 1 big tanker, you lose a boom. Even if doing the numbers, you lose 0.4 of a boom, you've lost a boom. Sure, you might have gained more fuel or even cargo, but you lost that boom. And as much as this ole tanker bubba wants to think tankers are more than just tankers (cargo, pax, etc.), it is all about the boom.
My background: I've worked on all sides of the tanker side of the house, was assigned to a Fighter Wing/Air Force where I was in charge with scheduling/requesting tankers, and now work in the AOC where I either build or execute the tanker plan of the day.
As a planner, a KC-135 sized tanker is great. It is light enough to refuel almost anything and still be compatible speed-wise with the receiver. The KC-10 has issues if it is heavy. I would suspect so would a 767 or A330. Additionally, a KC-10 gets limited to 15 degrees of bank when it is moderately loaded with fuel and has to refuel our Navy/NATO brothers and C-130s. Trying to be tactical in a KC-10 just isn't happening in that type of environment.
The USAF needs a mixture of tactical and strategic tankers. I'd like to see the USAF rebuild the KC-135. It is a very efficient and carries a large fuel load for it's size.