The optical rest-frame sizes of 10 of the most massive (∼5 × 1011 h−270 M⊙) galaxies found in the near-infrared MUNICS survey at 1.2 < z < 1.7 are analysed. Sizes are estimated in both the J and K′ filters. These massive galaxies are at least a factor of 4+1.9−1.0 (±1σ) smaller in the rest-frame V-band than local counterparts of the same stellar mass. Consequently, the stellar mass density of these objects is (at least) 60 times larger than that of massive ellipticals today. Although the stellar populations of these objects are passively fading, their structural properties are rapidly changing since that redshift. This observational fact disagrees with a scenario where the more massive and passive galaxies are fully assembled at z∼ 1.4 (i.e. a monolithic scenario) and points towards a dry merger scenario as the responsible mechanism for the subsequent evolution of these galaxies.