At around 20 mph, aero drag becomes a bigger factor than rolling resistance. So small wheels contribute to higher top speeds. Smaller wheels are also lighter weight, and lighter wheels make a bike accelerate faster. Recumbents are now being manufactured in sufficient quantities that tire makers have started making high quality tires in suitable sizes for recumbents. As a result, Rotator is using the ERTO 406 rim size as a standard on its bikes.
Most recumbent designs use a small front, and a large rear wheel.
Since the rear wheel is behind the rider's body, aerodynamic drag is not as much of a concern. We solved the gearing problem by developing a mid-drive system with two freewheels. Second, designing a performance recumbent requires a very wide gearing range. The mid-drive system has 35 speeds, so there is a good compromise between range and step. Mid-drive allows fully indexed shifting over all 35 combinations of gearing, too.