I'm not sure that 3 gal. is way bigger (we have the FC25-RBT 31-blk, 34-grey ~9%). Besides my black tank is always 'pre-charged' with about 3 gals to prevent pyramids.
When we boondock or have days between pump-outs we will use plastic bowls in the bath and kit sinks. If need be, they get dumped into the black tank.
It's good to have a lot of water in the black tank.
It may also have to do with load distribution &/or plumbing for any given floor-plan.
Retired: DEC et al