If you are (speed) force carbing then it probably helps since it will be easier to get a large surface area for the liquid to absorb Co2 (just like sloshing a full carboy vs. one with 1 gallon). If you are doing set and forget, it may take the same amount of time. Either way it won't hurt anything. You'll use less Co2 to force carb 1 gallon, but you'll use the same amount to serve 1 gallon as you would to serve 5 gallons (Since either way the keg is filled with ~12 psi of Co2 when you blow the keg)
Make sure you purge the keg well with Co2 before carbing though as you'll be starting out with more oxygen in the keg since there are ~4 gallons of head space.
You could also buy a "Carbonator cap" that fits onto 2L soda bottles and has a corny post on the other side so you can force carb a 2L instead of using a keg.