The structure of a brown dwarf is set by
degeneracy pressure.  Unlike a star, where the mass
sets both the radius and the photospheric temperature,
a brown dwarf has a radius and temperature that is nearly
independent of its mass.  All brown dwarfs are about
the same size as Jupiter.  The photospheric temperature
of a brown dwarf is set by its age, although the lifetime
of a brown dwarf is set by the mass.  Because the
low-mass brown dwarfs cool much faster than the
high-mass brown dwarfs, infrared surveys preferentially
find the more-massive brown dwarfs.