I do not have any particular aircraft model numbers in mind, but I know when watching youtube videos about passenger aircraft models, manufacturers tend to have “stretch” version of an aircraft where all else being equal, they basically tack on extra length somewhere in the fuselage and allow for more seats. This always results in a reduced range. This, on the surface, is logical as the, now heavier, plane requires more fuel to go the same distance. But why can’t they include an additional fuel tank in the belly of the aircraft which was just added? Seems like this would allow a longer aircraft to also have a longer range. And to add merit to my thought, the 737-700ER is no different than the 737-700 except that it has nine auxiliary fuel tanks increasing its range by 65%.
Edit: Turns out I was way oversimplified the 737-700ER. It is a much more complicated update.
Answer TL;DR: Increased Angle of Attack