The time evolution of `fiducial radio sources derived from fitting the dynamical model of Kaiser et al. (1997) is compared with the observational data for the `clan sources found in the sample of giant and normal-size FRII-type sources published Paper I (Machalski et al. 2004). Each `clan comprises 3, 4 or 5 sample sources having similar values of the two basic physical parameters: the jet power Q_{0} and central density of the galaxy nucleus rho_{0} (determined in Paper I) but different ages, radio luminosities and axial ratios. These sources are considered as the `same source observed at different epochs of its lifetime and used to fit the evolutionary luminosity-size (P-D) and energy density-total energy (u_{c}-E_{tot}) tracks derived from the model for a `fiducial source with Q0 and rho_{0} equal to the means of relevant values obtained for the `clan members, as well as to constrain the evolutionary model of the source dynamics used. In the result we find that (i) The best fit is achieved when the Kaiser et al.s model is modified by allowing an evolution of the sources cocoon axial ratio with time as suggested by Blundell et al. (1999). (ii) A slow acceleration of the average expansion speed of the cocoon along the jet axis is suggested by the `clan sources. We argue that this acceleration, although minor, may be real and some supporting arguments come from the well known hydrodynamical considerations.
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