The luminosity of a black body depends very strongly on temperature. Consider a square meter of the surface of a black body. To find the total luminosity of a black body, multiply the luminosity per square meter by the number of square meters on its surface. As another example, Sirius has a dim companion, which can be seen through a reasonably large telescope. The reason why the companion to Sirius has such a low total luminosity is that it's tiny - even smaller than the Earth.
Stars have a wide range of radii.
The largest known star has a radius 3700 times that of the Sun.