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 (i₇₇₅, z₈₅₀, Y₁₀₅, J₁₂₅, H₁₆₀), and find that the size decreases by ~20 per cent going from i₇₇₅ to H₁₆₀ band, consistent with recent studies. We then generate spatially resolved stellar mass maps using an empirical relationship between M_*/L_H160 and (z₈₅₀- H₁₆₀) 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_*/L_H160 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.