There are tons of small turbos. Turbos are used in other applications. There is a plant here in Indianapolis that makes very small and very large turbines. It's just that for the small ones, the turbine is often powered by an electric motor. But the purpose can still be to boost pressure in combustion chambers (cylinders in cars). I have no idea what the application is for the ones they make, but I just did a briefing for an elected official out here who was visiting the plant. There are four-stroke marine motors that have small turbos on them.
It would be interesting to see if the exhaust from the small cars would be enough to power a turbine. I guess it's all relative to size. I would think that the friction coefficient of any lubrication and the weight of the turbo materials, the effects of which are negligible in a large motor, would make a much larger difference. It's a lot harder for a 10cc motor to push materials through a lubricant than a 1900 cc one. and spooling up even aluminum would be a hefty task for a car that as a whole weighs less than 10 lbs.
