Early-type galaxies (ETGs) are observed to be more compact (smaller size and higher velocity dispersion) at higher redshift. We compare this observed evolution with the evolution expected in LambdaCDM cosmology. We find that theory and observations are consistent up to redshift 2, while there is significant tension at higher z, in the sense that real galaxies evolve faster than expected. At lower redshift further observational constraints derive from the measurement, in lens ETGs, of the total mass density slope gamma', which is found to be almost constant at redshift between 0 and 1. We construct a theoretical model for the joint evolution of size and gamma' driven by mergers occurring at rates given by cosmological simulations. When compared with the observations, this model supports a scenario in which the outer regions of massive ETGs grow by accretion of stars and dark matter, while small amounts of dissipation and nuclear star formation conspire to keep the mass density profile constant and approximately isothermal.