Chains running over smaller cogs and smaller gear combos, lose a little bit of efficiency compared to larger cogs, IMO. You don't see track riders running 10 or 11 tooth cogs much. Plus smaller cogs need to turn faster so they wear out quicker.
You would think if it were advantageous to go really small, that bike or car designers would have designed tiny transmissions the size of a cigarette box, in order to save weight. But they don't, for whatever reason, I suspect efficiency and premature wear.