There are 3 planned tanker programmes over the next 30 years,
KC-X, KC-Y and KC-Z. These will initially replace the KC-135, then the KC-10.
The design for the KC-X was left somewhat open to allow the best, as the USAF saw it, proposal to win. If it was a larger jet, then the selection in the KC-Y or Z competitions could be scoped to give the best fleet size/number mix. Buy small now, large later; large now, larger later etc, depending on the need in 20 years time (small wars, tactical tankers; threat from China, large tankers to drag formations and support across the Pacific etc).