We present 0"2 resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations at 870 μm in a stellar mass-selected sample of 85 massive ( ${M}_{\star }\gt {10}^{11}\,{M}_{\odot }$ ) star-forming galaxies (SFGs) at $z=1.9\mbox{--}2.6$ in the CANDELS/3D-Hubble Space Telescope 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 ${R}_{{\rm{e}},\mathrm{FIR}}=0.4\,\mathrm{kpc}$ to ${R}_{{\rm{e}},\mathrm{FIR}}=6\,\mathrm{kpc}$ . The effective radius of the FIR emission is smaller by a factor of ${2.3}_{-1.0}^{ 1.9}$ than the effective radius of the optical emission and is smaller by a factor of ${1.9}_{-1.0}^{ 1.9}$ than the half-mass radius. Taking into account potential extended components, the FIR size would change only 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, ${M}_{1\mathrm{kpc}}$ . The compact starburst puts most of the 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 ${M}_{1\mathrm{kpc}}={10}^{10.5}\,{M}_{\odot }$ , 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 μm and James Webb Space Telescope 3-4 μm will explicitly verify this scenario.
