  We explore radio and spectroscopic properties of a sample of 14 miniature
radio galaxies, i.e. early-type core galaxies hosting radio-loud AGN of
extremely low radio power, 10^(27-29) erg s^(-1) Hz^(-1) at 1.4 GHz. Miniature
radio galaxies smoothly extend the relationships found for the more powerful
FRI radio galaxies between emission line, optical and radio nuclear
luminosities to lower levels. However, they have a deficit of a factor of ~100
in extended radio emission with respect to that of the classical example of
3CR/FRI. This is not due to their low luminosity, since we found radio galaxies
of higher radio core power, similar to those of 3CR/FRI, showing the same
behavior, i.e. lacking significant extended radio emission. Such sources form
the bulk of the population of radio-loud AGN in the Sloan Digital Sky Survey.
At a given level of nuclear emission, one can find radio sources with an
extremely wide range, a factor of >~100, of radio power. We argue that the
prevalence of sources with luminous extended radio structures in flux limited
samples is due to a selection bias, since the inclusion of such objects is
highly favored. The most studied catalogues of radio galaxies are thus composed
by the minority of radio-loud AGN that meet the physical conditions required to
form extended radio sources, while the bulk of the population is virtually
unexplored.
