I had read previously that larger chainrings and cogs are slightly more efficient. It has to do with the chain changing direction, and the greater bending of the links around the smaller radiuses of the smaller cogs and chainrings. Those of you who have been doing this for a while will have noted that derailleur jockey pulleys keep getting bigger and bigger......

Brett 12 and I have previously discussed the cogging effect of 11T rear cogs, v. larger ones. Apparently as the chain leaves the top of the cog to return to the chainring, there is variation in ratio (?) as the chain bears on different parts of the tooth, which reduces efficiency, this effect occurring only on cogs of 11T or less with 1/2" pitch chain. I knew enough to mention it and name it properly ("cogging effect"), and note that you can feel some vibration in the drivetrain in the 11T that is not present in the 12T, but I still haven't read up on it, because that exceeds my need for knowledge on the subject. If true, it certainly marks a point at which there is a stepwise reduction in efficiency of smaller cogs.