  We report the source size distribution, as measured by ALMA millimetric
continuum imaging, of a sample of 13 AzTEC-selected submillimeter galaxies
(SMGs) at z_photo ~ 3-6. Their infrared luminosities and star-formation rates
(SFR) are L_IR ~ 2-6 x 10^12 L_sun and ~ 200-600 M_sun yr-1, respectively. The
size of z ~ 3-6 SMGs ranges from 0".10 to 0".38 with a median of
0".20+0".03-0".05 (FWHM), corresponding to a median circularized effective
radius (Rc,e) of 0.67+0.13-0.14 kpc, comparable to the typical size of the
stellar component measured in compact quiescent galaxies at z ~ 2 (cQGs) --- R
~ 1 kpc. The median surface SFR density of our z ~ 3-6 SMGs is 100+42-26 M_sun
yr-1 kpc-2, comparable to that seen in local merger-driven (U)LIRGsrather 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.
