The more vacuum actuated items you have the more volume you need in a vacuum reservour for them to work reliably and immediately. Like mentioned, the bigger vacuum reservoirs (cylinder or pill shaped) are for vehicles with cruise control, the smaller ones (sphere or ball shaped) are for vehicle with NO cruise control, just the vacuum activated HVAC duct doors and heater core valve that every XJ has.
My '95 has NO cruise control, but does have the Cylinder (Pill) shaped reservoir. Maybe at some point they just switched to the bigger reservoir, instead of carrying two different types. Having more volume than you need doesn't hurt.
I had a little Plymouth Sundance that the HVAC ducts would NOT always change over after pressing the buttons, I'd have to lift off the accelerator pedal (increase vacuum in the motor) to get the HVAC duct doors to actually move. I could NOT find any sign of a vacuum leak, but it didn't have a vacuum reservoir (cheap economy car, every cost was minimized in production), but probably needed one.