Hybrid makers are making vehicles that are too powerful is all. Hondas Insight is the only one over here that gets really good mileage, 70mpg+ with American gallons.
The Prius and Civic arent that small, and yanks demand power as they dont know how to wring it out of a small engine.
The other vehicles getting the treatment are big luxury SUVs which use the tech to get more performance, not better mpg.
Im sure it would be possible to build a 120mpg Euro style supermini with diesel/hybrid tech. Give them time.