We present Gran-Telescopio-Canarias/OSIRIS optical spectra of four of the most compact and massive early-type galaxies (ETGs) in the Groth Strip Survey at redshift z ~ 1, with effective radii R e = 0.5-2.4 kpc and photometric stellar masses M sstarf = (1.2-4) × 1011 M sun. We find that these galaxies have velocity dispersions σ = 156-236 km s-1. The spectra are well fitted by single stellar population models with approximately 1 Gyr of age and solar metallicity. We find that (1) the dynamical masses of these galaxies are systematically smaller by a factor of ~6 than the published stellar masses using BRIJK photometry, and (2) when estimating stellar masses as 0.7× M dyn, a combination of passive luminosity fading with mass/size growth due to minor mergers can plausibly evolve our objects to match the properties of the local population of ETGs.