The C-17 is a big fuel guzzler.
A C-17 can carry 45.t over a distance of 4,000 nm. An A330F can carry other such a distance 64 t. By splitting the range you can estimate how much fuel each aircraft can deliver at a range of 2,000 nm.
MTOW of a C-17 is also 30 t heavier than A330.
According to Air Force data a C-17 burns about 103 % more fuel than a B767 or about 65 % more than a A330.
If you really like a cargo aircraft why not AN-124?
Can deliver 92 t at 4,000 nm but guzzles about 50 % more than a C-17. But even AN-124 needs less fuel for each pound of fuel delivered than a C-17.
Try flying an A330F into or out of an unimproved 3,000’ runway. Try getting a Stryker vehicle into an A330F. But if all you’re moving is palletized cargo, it is a good choice, which is why it would make an excellent KC-X.
If the AN-124 isn’t full, it’s fuel per pound of delivered cargo goes way up.
The A400M is a nice compromise between the C-130 and the C-17, but cost and delivery times are uncertain.
There is no one size fits all in the cargo business.