© 2015. The American Astronomical Society. All rights reserved. We report the source size distribution, as measured by ALMA millimetric continuum imaging, of a sample of 13 AzTEC-selected submillimeter galaxies (SMGs) at zphot ∼ 3-6. Their infrared luminosities and star formation rates (SFRs) are LIR∼ 2-6 × 1012 L⊙ and ∼200-600 M⊙ yr1, respectively. The sizes of these SMGs range from 0″ 10 to 0″ 38, with a median of 0″ 20 -0″05+0″03 (FWHM), corresponding to a median circularized effective radius (Rc,e) of 0.67-0.14+0.13 kpc, comparable to the typical size of the stellar component measured in compact quiescent galaxies at z ∼ 2 (cQGs)-Re ∼ 1 kpc. The median surface SFR density of our SMGs is 100-26+42 M yr1 kpc2, comparable to that seen in local merger-driven (U)LIRGs rather than in extended disk galaxies at low and high redshifts. The discovery of compact starbursts in z ≳ 3 SMGs strongly supports a massive galaxy formation scenario wherein z ∼ 3-6 SMGs evolve into the compact stellar components of z ∼ 2 cQGs. These cQGs are then thought to evolve into the most massive ellipticals in the local universe, mostly via dry mergers. Our results thus suggest that z ≳ 3 SMGs are the likely progenitors of massive local ellipticals, via cQGs, meaning that we can now trace the evolutionary path of the most massive galaxies over a period encompassing ∼90% of the age of the universe.