Bigger how? From the Air Force Factsheet for the KC-10 Extender:
In addition to the three main DC-10 wing fuel tanks, the KC-10 has three large fuel tanks under the cargo floor, one under the forward lower cargo compartment, one in the center wing area and one under the rear compartment. Combined, the capacity of the six tanks carry more than 356,000 pounds (160,200 kilograms) of fuel - almost twice as much as the KC-135 Stratotanker.The large cargo-loading door can accept most air forces' fighter unit support equipment. Powered rollers and winches inside the cargo compartment permit moving heavy loads. The cargo compartment can accommodate loads ranging from 27 pallets to a mix of 17 pallets and 75 passengers.
From the Northrup Grumman KC-45 website:
Max. fuel capacity 25% more than KC-135 [Northrup site shows the KC-135 to be 200,000lbs, so that would be 250,000lbs. -Yo]Passengers/Troops: 226 Pallets: Up to 32 463L pallets.
So the KC-45A may hold more cargo and troops, but carrys 30% less fuel than the KC-10A. Part of the difference is that the KC-10 is set up only for a combi mode of passengers and cargo, not all passengers like the KC-45 figure. It is yet to be seen if the Air Force actually fits out the KC-45 to accept an all passenger layout option, or if it remains a combi layout.
Part of the appeal of a large tanker/freighter is that you can escort the flight of fighters to the remote zone, bring along the ground crew, and bring the spares kits. Doing that requires a combi mode, so the real passenger load may be much less than 226.
The KC-10 is also an earlier generation aircraft, so just because it carries 30% more fuel, that doesn't necessarily translate to 30% more offload capability. Doing the range vs. fuel offload capability math can be left to others with more patience.
I know I said the KC-Y was to replace the KC-10, but I was partly wrong. The KC-Y will be an evaluation point after the first 179 KC-45As are in service, to determine if another bid needs to go out, or to simply buy more of the same. It may be that the Air Force decides to buy more KC-45s to replace the rest of the KC-135s, and bid a larger aircraft to replace the KC-10. The "KC-777", which is still very much a paper aircraft at this point, still would be larger than the KC-45 and carry more cargo, if not more fuel.
The KC-Z will most likely be the next generation blended wing that everyone (including me) is so hot for these days, but has it's own very long term development challenges.