I might be wrong but it was my understanding that in general a bigger prop was *more* effecient. You go to smaller props because:

- You can spin the prop because of ground clearance
- You are after pitch speed more than thrust

If you want longer flight duration reduce the throttle - I think going to a smaller prop will probably make it worse.

Reducing or increasing prop size is typically about either increasing top speed and pitch speed, or increasing thrust - flight duration is a different topic.