  We present 0.2arcsec-resolution Atacama Large Millimeter/submillimeter Array
observations at 870 $\mu$m in a stellar mass-selected sample of 85 massive
($M_\mathrm{star}>10^{11}~M_\odot$) star-forming galaxies (SFGs) at z=1.9-2.6
in the 3D-HST/CANDELS fields of UDS and GOODS-S. We measure the effective
radius of the rest-frame far-infrared (FIR) emission for 62 massive SFGs. They
are distributed over wide ranges of FIR size from $R_\mathrm{e,FIR}=$0.4 kpc to
$R_\mathrm{e,FIR}=$6 kpc. The effective radius of the FIR emission is smaller
by a factor of 2.3$^{+1.9}_{-1.0}$ than the effective radius of the optical
emission and by a factor of 1.9$^{+1.9}_{-1.0}$ smaller than the half-mass
radius. Even with taking into account potential extended components, the FIR
size would change by ~10%. By combining the spatial distributions of the FIR
and optical emission, we investigate how galaxies change the effective radius
of the optical emission and the stellar mass within a radius of 1 kpc,
$M_\mathrm{1kpc}$. The compact starburst puts most of massive SFGs on the
mass--size relation for quiescent galaxies (QGs) at z~2 within 300 Myr if the
current star formation activity and its spatial distribution are maintained. We
also find that within 300 Myr, ~38% of massive SFGs can reach the central mass
of $M_\mathrm{1kpc}=10^{10.5}~M_\odot$, which is around the boundary between
massive SFGs and QGs. These results suggest an outside-in transformation
scenario in which a dense core is formed at the center of a more extended disk,
likely via dissipative in-disk inflows. Synchronized observations at ALMA 870
$\mu$m and JWST 3-4 $\mu$m will explicitly verify this scenario.
