Several things going on here:
stress level and material properties - contact pressures, loads
First point, it is not just about the number of teeth. If you use a different gear modulus then you can have the same diameter contrate with different number of teeth. Just look at some of the early brass gears, fine pitch without large diameter.
The error robusness is the bit that really determines the size of teeth that should be used. Misalignment and tolerances of build need larger teeth. A small inaccuracy should not lead to the gears disengaging and jumping teeth.
Too large teeth gives a pinion with too few teeth, you don't really want less than 8, the involute profile is compromised and you end up with more of a sliding action as the teeth engage and rotate, so friction gets higher and transmission losses increase, noise, heat, wear all the cost to you of speed and power.
Plastic teeth need a reasonable cross section to resist the high torques of modern motors. This can be achieved by increasing the depth of the tooth, but again that can lead to more rubbing. Look at the gears on the AutoArt cars, same pitch but really thin. Put a NSR motor in one of those cars and the gears are going to fail really quickly...
So 9:27 gives decent size plastic teeth, allows for reasonable mismatch, gears are not too large and the tranmission efficiency is good.