Description: We analyse the sizes, colour gradients and resolved stellar mass distributions for 36 massive and passive galaxies in the cluster XMMUJ2235-2557 at z  = 1.39 using optical and near-infrared Hubble Space Telescope ( HST ) imaging. We derive light-weighted Sérsic fits in five HST bands ( i 775 , z 850 , Y 105 , J 125 , H 160 ), and find that the size decreases by ~20 per cent going from i 775 to H 160 band, consistent with recent studies. We then generate spatially resolved stellar mass maps using an empirical relationship between $M_{{\ast }}/L_{H_{160}}$ and ( z 850  –  H 160 ) and use these to derive mass-weighted Sérsic fits: the mass-weighted sizes are ~41 per cent smaller than their rest-frame r -band counterparts compared with an average of ~12 per cent at z  ~ 0. We attribute this evolution to the evolution in the $M_{{\ast }}/L_{H_{160}}$ and colour gradient. Indeed, as expected, the ratio of mass-weighted to light-weighted size is correlated with the M * / L gradient, but is also mildly correlated with the mass surface density and mass-weighted size. The colour gradients ( z  –  H ) are mostly negative, with a median value of ~0.45 mag dex –1 , twice the local value. The evolution is caused by an evolution in age gradients along the semimajor axis ( a ), with age  = dlog ( age )/dlog ( a ) ~– 0.33, while the survival of weaker colour gradients in old, local galaxies implies that metallicity gradients are also required, with Z  = dlog ( Z )/dlog ( a ) ~– 0.2. This is consistent with recent observational evidence for the inside–out growth of passive galaxies at high redshift, and favours a gradual mass growth mechanism, such as minor mergers.