Because horsepower is a function of torque and rotational speed, I'm guessing these cars don't have a lot of torque but make up for it by screaming high RPM, that's why I said that. I'm assuming they have the components to support running at that RPM, including gearing to make it feasible on the street.
Hell, Indy cars make twice that much horsepower with just a liter more (3.5L for those not savvy) , but they rev limit at 10300 RPM. Any size engine can make crazy HP numbers providing you can flow enough air and fuel at the speed you need to spin the crank.