Three open problems in our understanding of early-type galaxies are 1) identifying the process(es) responsible for their rapid size evolution, 2) accurately constraining the stellar IMF and its variations in the population, 3) measuring the density profile of their dark matter halo. We use strong lensing as the main diagnostic tool to address these issues. We first dissected a massive elliptical galaxy in its stellar and dark matter components, measuring both its IMF and the inner slope of the dark matter halo. We then collected a sample of 45 strong lenses in the redshift interval 0.2 < z < 0.8 and used them, in combination with lenses from other surveys, to measure the slope of the total density profile, the stellar IMF and the dark matter mass in the population of massive early-type galaxies, and their time evolution. Finally, we used our measurements of the evolution of the density slope to test a galaxy growth scenario based on purely dissipationless mergers. Our main results are: the stellar IMF of massive early-type galaxies is significantly heavier than that of the Milky Way and correlates with galaxy mass; the dark matter halo has a steep slope in at least one system; more compact galaxies have less dark matter than their extended counterparts at fixed redshift and stellar mass; early-type galaxies evolve while keeping the slope of their total density profile approximately constant. This last result cannot be reproduced with purely dissipationless mergers, therefore a little amount of dissipation is required.