  The recent discovery, by the Chandra satellite, that jets of blazars are
strong X-ray emitters at large scales (0.1-1 Mpc) bears support to the
hypothesis that (also) on these scales the emitting plasma is moving at highly
relativistic speeds. In this case in fact the emission via inverse Compton
scattering off cosmic background photons is enhanced and the resulting
predicted X-ray spectrum well accounts for the otherwise puzzling observations.
Here we point out another reason to favor relativistic large scale jets, based
on a minimum power argument: by estimating the Poynting flux and bulk kinetic
powers corresponding to, at least, the relativistic particles and magnetic
field responsible for the emission, one can derive the value of the bulk
Lorentz factor for which the total power is minimized. It is found that both
the inner and extended parts of the jet of PKS 0637-752 satisfy such condition.
