  The energetic composition of radio lobes in the FR II galaxies $-$ estimated
by comparing their radio luminosities with the powers required to inflate
cavities in the external medium $-$ seems to exclude the possibility of their
energetic domination by protons. Furthermore, if the jets were dominated by the
kinetic energy of cold protons, it would be difficult to efficiently accelerate
leptons in the jets' terminal shocks. Assuming that the relative energy
contents of leptons, protons and magnetic fields are preserved across the
shocks, the above implies that the large-scale jets should also be
energetically dominated by leptons: $P_{\rm e,j} \gtrsim P_{\rm p,j}$. On the
other hand, previous studies of small-scale jets in blazars and radio cores
suggest a pair content (number of electrons and positrons per proton) of the
order of $n_{\rm e}/n_{\rm p} \sim 20$. Assuming further that the particle
composition of jets does not evolve beyond the blazar scales, we show that this
implies an average random Lorentz factor of leptons in large-scale jets of
$\bar\gamma_{\rm e,j} \gtrsim 70(1+\chi_{\rm p})(20n_{\rm p}/n_{\rm e})$, and
that the protons should be mildly relativistic with $\chi_{\rm p} \equiv
(\epsilon_{\rm p} + p_{\rm p})/\rho_{\rm p} c^2 \lesssim 2$, $p_{\rm p}$ the
pressure of protons, $\epsilon_{\rm p}$ the internal energy density of protons,
and $\rho_{\rm p} c^2$ the rest-mass energy density of protons. We derive the
necessary conditions for loading the inner jets by electron-positron pairs and
proton-electron plasma, and provide arguments that heating of leptons in jets
is dominated by magnetic reconnection.
