The mechanisms responsible for the remarkably small sizes of the most massive quiescent galaxies at z~2 are poorly understood. Partly because the nature of their progenitors is still unknown. Barro+13 used the deepest CANDELS WFC3/F160W data to identify, for the first time, a population of massive compact star-forming galaxies (cSFGs) at 2 < z < 3, whose structural properties and number densities suggest that they are the progenitors of such population. cSFGs present spheroidal morphologies and centrally-concentrated mass profiles similar to those of quiescent galaxies. However, according to their optical/NIR (UVJ) and far-IR (Spitzer/Herschel) colors, they are strongly star-forming and heavily dust obscured. In Barro+14, we modeled the stellar populations of ~50 cSFGs, using extremely detailed SEDs, finding that cSFGs have stellar masses ranging from M_star = 10^[10.3-11.2] M_sun and maximally old stellar ages of ~2.4 Gyr. Interestingly, we also find that low-mass cSFGs have younger ages but lower sSFR than the more massive cSFGs. This suggest that the most massive galaxies had longer SFHs, contrary to the downsizing intuition.