Chordal angle. I have a chart somewhere that shows just how much the chordal angle decreases as the tooth count on the sprocket increases. The number is pretty big for the 9 – 10 – 11 tooth sprockets, but gets increasingly smaller as a tooth count goes up. Look it up on the web, maybe Wikipedia. I don't know where my copy is, I have a big hard disk with lots of files.
Don't know for a fact, but as little as the angle changes between the bigger drivers and axle sprockets, I wonder how much difference it could make. I think, just a theory, that the changes in chain speed have more to do with it.
I'm kind of wondering why nobody has done a definitive study on this phenomena? I'm pretty sure it could be measured with the proper equipment.