I was looking at gearinch for 700-38 vs 650-50 and it seems the gearinch difference would be the equivalent of 1.25T on the chainring so if you were to swap from a 40T chainring to a 39T, you 'd get a similar effect.
Now,
Badger6 comment about "spinning up" faster. That is interesting as I noticed the same and thought it was down to better rolling resistance of the tyres i have on the 650b.
Now, you could argue the "heavier" outter of the 650B will require more torque to change rpm drastically but will store more energy like so should keep spinning at a given rpm range better like a flywheel.
Not sure if it is really noticeable. (I think DT swiss published something about than).