Once tank size gets to a certain point (the "range" of the driver/passengers for example) the only reason to get bigger is to reduce refueling time or because of fuel availability limitations.

This is part of the reason super long flights aren't as common as might be expected, because it's often significantly cheaper to have a layover, even if the plane could actually do the flight in one go - having to carry the second half fuel for the first half is heavy (and the extreme version of this is the rocket equation).