When Shimano came out with the 12T cog (or maybe it was the 11T), they had one engineer working solely on optimizing the shape of the cog. Since a smaller count cog will be less round (imagine a "1T cog"), you lose roundness as the tooth count drops. Below 14 or 15T it gets pretty significant, i.e. pulley wheel size.

But between chainrings.

This is why you rarely see 10T cogs (like on the folding small wheel Moulton bikes) on regular road bikes. You could use smaller rings with the 10T, saving weight, but it's less efficient.

It's also why Shimano went to the 1 cm drive (10mm) versus the slightly longer but standard 1/2" drive. This refers to the distance between chain pins. 10mm is slightly less than the 12.2mm of the 1/2" drive, and it was enough to significantly reduce the size of the cogs without losing roundness. Shimano could make much smaller cogs, a smaller chain, and save weight overall. Since they didn't do anything with shifting (lateral loads on chain etc) this was made just for track bike use. The System 10 is still a sought after drivetrain for trackies and collectors.